Medical Definition of THEMATIC APPERCEPTION TEST. : a projective psychological test that is used in clinical psychology to make personality, psychodynamic, and diagnostic assessments based on the subject's verbal responses to a series of black and white pictures—abbreviation TAT. a projective psychological test that is used in clinical psychology to make personality, psychodynamic, and diagnostic assessments based on the subject's verbal responses to a series of black and white pictures—abbreviation TAT.