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60 entries explaining psychology concepts, theories and measures in plain language, each with its own page and references. Free to read, no account needed.
Counted from the PsychologyOrbit knowledge base on 2 November 2026.
A
Acceptance and Commitment Therapy
A behavioural therapy that works on the relationship a person has with their thoughts and feelings rather than on the content of those thoughts, aiming at psychological flexibility and action guided by personal values.
ADHD
A neurodevelopmental condition involving persistent inattention, hyperactivity and impulsivity at a level that interferes with functioning, present before age 12 and across more than one setting.
Adolescent Brain Development
The prolonged reorganisation of the brain through the second decade of life, involving synaptic pruning, continued myelination and shifting balance between reward-sensitive and control systems.
Adverse Childhood Experiences
A set of childhood exposures — abuse, neglect and household dysfunction — that show a graded association with poorer health, mental health and social outcomes in adulthood. Widely adopted in policy, and widely misused at the individual level.
Attachment Theory
An account of the biologically-based system that leads infants to seek proximity to a caregiver under threat, and of how repeated experience with that caregiver shapes expectations about closeness, distress and dependence.
Autism
A lifelong neurodevelopmental difference involving differences in social communication and interaction alongside restricted or repetitive patterns of behaviour, interests or activities, and differences in sensory processing.
B
Big Five Personality Traits
A five-factor description of personality — openness, conscientiousness, extraversion, agreeableness and neuroticism — derived from the structure of trait language and replicated across many samples and instruments.
Bipolar Disorder
A mood disorder marked by episodes of mania or hypomania — elevated or irritable mood with increased energy — usually alongside episodes of depression. It is highly treatable, but often misdiagnosed as depression for years.
Borderline Personality Disorder
A diagnosis describing a pervasive pattern of instability in relationships, self-image and affect, with marked impulsivity, frequent self-harm and intense fear of abandonment. The label itself is contested.
Burnout
A syndrome of emotional exhaustion, cynicism and reduced professional efficacy arising from chronic workplace stress that has not been successfully managed. Classified as an occupational phenomenon, not a medical condition.
Bystander Effect
The finding that a person is less likely to help in an emergency when other people are present. Its textbook origin story is largely inaccurate, and real-world footage shows that in public conflicts someone usually does intervene.
C
Classical Conditioning
A form of learning in which a neutral stimulus, repeatedly paired with a stimulus that already produces a response, comes to produce a related response by itself.
Cognitive Behavioural Therapy
A structured, time-limited psychological treatment that works on the relationship between thoughts, behaviour and emotion, usually over 6 to 20 sessions with tasks between them.
Cognitive Dissonance
The discomfort produced by holding inconsistent cognitions, or by acting in a way that contradicts a belief, and the motivated change in attitude or behaviour that follows from trying to reduce it.
Cognitive Load Theory
An instructional theory holding that learning is constrained by working memory capacity, and that teaching should minimise processing that does not contribute to building knowledge in long-term memory.
Conformity
Changing behaviour or judgement to match a group, whether because the group is taken as evidence about reality or because of the social cost of standing apart.
Construct Validity
The degree to which a measure actually captures the theoretical construct it is intended to capture, established by an accumulating argument rather than by any single coefficient.
D
Dialectical Behaviour Therapy
A structured treatment combining individual therapy, group skills training and between-session coaching, developed for chronically suicidal and self-harming clients and built around balancing acceptance with change.
Dissociation
A disruption in the normally integrated functions of consciousness, memory, identity, emotion, perception or sense of self, ranging from everyday absorption to the compartmentalised identity states of dissociative identity disorder.
Dual-Process Theory
A family of accounts proposing that reasoning and judgement draw on two kinds of processing: fast, automatic and intuitive, and slow, effortful and deliberative.
E
Effect Size
A quantitative measure of the magnitude of a phenomenon, independent of sample size. It answers how big, where a significance test answers only whether an effect can be distinguished from zero.
EMDR
Eye Movement Desensitisation and Reprocessing: a structured trauma treatment in which the client holds a traumatic memory in mind while performing a bilateral task, usually tracking the therapist's hand with their eyes.
Emotion Regulation
The processes by which people influence which emotions they have, when they have them, and how they experience and express them — a capacity that develops through childhood and predicts mental health across the lifespan.
Exposure Therapy
A behavioural treatment in which a person deliberately and repeatedly faces a feared situation, memory or sensation without the escape behaviours that normally end the fear, until the fear response changes.
F
G
H
I
Intergroup Contact Hypothesis
The idea that contact between members of different groups reduces prejudice, especially when it is cooperative, between equals, and supported by authorities. It is one of the best-supported ideas in social psychology, with important limits.
Item Response Theory
A family of models describing the probability of a given response to an item as a function of the respondent's standing on a latent trait and the properties of that item, rather than as a simple summed score.
J
L
M
Major Depressive Disorder
A condition defined by at least two weeks of persistently low mood or loss of interest, together with changes in sleep, appetite, energy, concentration and self-worth, causing significant impairment.
Measurement Invariance
The property that an instrument measures the same construct in the same way across groups or across time. Without it, a difference between groups may reflect how they interpret the items rather than any real difference.
Memory Reconsolidation
The finding that retrieving a consolidated memory can return it to an unstable state in which it must be restabilised, opening a window in which it can be modified or weakened.
Motivational Interviewing
A collaborative conversational method for strengthening a person's own motivation for change, by drawing out and reinforcing the arguments for change that they make themselves rather than supplying them.
O
Obedience to Authority
The tendency to comply with instructions from a perceived legitimate authority, including instructions that conflict with a person's own moral judgement. Studied most famously, and most contentiously, by Milgram.
Observational Learning
Learning a behaviour, and its likely consequences, by watching someone else perform it. Bandura showed that people acquire responses without ever being reinforced for them, which classical and operant conditioning alone could not explain.
Obsessive-Compulsive Disorder
A condition involving intrusive, unwanted thoughts, images or urges that cause marked distress, and repetitive behaviours or mental acts performed to reduce that distress or prevent a feared outcome.
Operant Conditioning
Learning in which the consequences of a behaviour change how likely that behaviour is to occur again, through reinforcement that increases it or punishment that decreases it.
P
P-Value
The probability of obtaining data at least as extreme as those observed, if the null hypothesis and every other assumption of the statistical model were true. It is not the probability that the hypothesis is false.
Piaget's Theory of Cognitive Development
Jean Piaget's account of children as active builders of knowledge who pass through four stages of thinking. The stages have not survived intact, but the idea that children reason differently rather than simply knowing less still shapes education.
Placebo Effect
A genuine change in a person's condition produced by the psychosocial context of a treatment — expectation, conditioning, ritual and the clinical relationship — rather than by any specific active ingredient.
Post-Traumatic Stress Disorder
A condition that can follow exposure to actual or threatened death, serious injury or sexual violence, characterised by intrusive re-experiencing, avoidance, negative changes in mood and thinking, and heightened arousal.
Preregistration
Specifying a study's hypotheses, design and analysis plan in a time-stamped, read-only record before the data are collected or examined, so that confirmatory claims can be distinguished from exploratory ones.
Psychodynamic Therapy
A family of therapies working with unconscious process, early relationship patterns, defence, and what happens between client and therapist, typically less structured and longer than behavioural treatments.
Psychological Safety
A shared belief within a team that it is safe to take interpersonal risks — to ask a question, admit a mistake, disagree or raise a concern — without being humiliated or penalised for it.
Publication Bias
The tendency for studies with significant or striking results to be published, and for null results to remain unpublished, which distorts the literature and inflates every estimate built from it.
R
Reliability
The consistency of a measurement — across items, occasions, raters or forms. It sets a ceiling on validity: a score that varies at random cannot measure anything, but a perfectly consistent score may still measure the wrong thing.
Replication Crisis
The finding, from the 2010s onward, that a substantial share of published psychological results do not replicate when repeated with adequate power and preregistered methods, and the reform movement that followed.
S
Schizophrenia
A serious mental disorder involving psychosis — hallucinations and delusions — together with disorganised thinking and reduced motivation and expression. It usually begins in late adolescence or early adulthood, and outcomes are far more varied and often better than its reputation suggests.
Self-Determination Theory
A theory of motivation holding that people function and grow best when three basic psychological needs are met — autonomy, competence and relatedness — and that the quality of motivation matters more than its quantity.
Self-Efficacy
A person's belief in their ability to carry out the actions needed to reach a particular goal. Introduced by Albert Bandura, it predicts effort, persistence and performance, and it is specific to a task rather than a general trait.
Social Identity Theory
An account of intergroup behaviour holding that people derive part of their self-concept from group membership, and are motivated to see their own groups as positively distinct from others.
Statistical Power
The probability that a study will detect an effect of a given size if that effect exists. Low power both misses real effects and, through selective publication, inflates the size of the effects that do get published.
Stereotype Threat
The proposal that awareness of a negative stereotype about a group one belongs to can impair performance on a task the stereotype concerns. Widely influential, and one of the areas where replication has substantially reduced confidence.
Stigma
The process by which a characteristic is labelled, linked to negative stereotypes, and used to separate and devalue people — producing discrimination, and operating at structural, public and internalised levels.
T
Theory of Mind
The capacity to attribute mental states — beliefs, desires, intentions, knowledge — to oneself and others, and to understand that another person can hold a belief that differs from reality and from one's own.
Therapeutic Alliance
The collaborative bond between therapist and client, comprising agreement on goals, agreement on tasks, and the affective bond. It is one of the most consistent correlates of outcome across every therapy studied.
W
Z
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