1. an innate, species-specific biological force that impels an organism to do something, particularly to perform a certain act or respond in a certain manner to specific stimuli. See also hormic psychology.

2. in classical psychoanalytic theory, a basic biological drive (e.g., hunger, thirst, sex, aggression) that must be fulfilled in order to maintain physical and psychological equilibrium. Sigmund Freud classified instincts into two types: those derived from the life instinct and those derived from the death instinct. See also component instinct; erotic instinct; libido; satisfaction of instincts; sexual instinct.

3. in popular usage, any inherent or unlearned predisposition (behavioral or otherwise) or motivational force. —instinctive or instinctual adj.