ultradian rhythms: one complete cycle that repeats in less than twenty-four hours, for instance different stages of sleep several times during a single night's sleep.

unconditional positive regard: complete acceptance and caring of an individual, without imposing conditions.

unconditioned response: in classical conditioning, a reflexive response elicited by an unconditioned stimulus, such as pupil contraction to bright light, without prior learning.

unconditioned stimulus: in classical conditioning, a stimulus which elicits a reflexive (unconditioned) response.

unconscious: in Freud's theory, portion of the psyche that cannot be directly accessed by the unconscious, repressing urges, impulses and thoughts, which may filter into conscious awareness directly or in symbolic form.

unconscious motive: a term used to describe that much of (motivated) behaviour is a result of influences outside our conscious awareness, and manifests in defence mechanisms or other symbolic ways.

understanding: the cognitive condition of someone who understands.  It is is the possession of knowledge coupled with the capability of reasoning and making judgements relating to the applicability of the knowledge.

unfalsifiable: a theory or hypothesis is unfalsifiable if it cannot be disproved by data and thus cannot be used to make predictions.

unipolar depression: see depression

universal: any characteristic that can be applied to all members of the species, despite a variety of experiences and development.

unstructured interview: an interview whereby the interviewer does not have pre-determined questions, but instead asks questions spontaneously as topics arise.

upper quartile: the data point that is at the 75 per cent point of the data set when the data is ranked in order.

utilitarianism: states that what is ethically acceptable is that which produces the greatest pleasure and happiness (in comparison to pain and suffering) for the greatest number of people.