This pseudo-Greek term was introduced by Freud's translator/editor James Strachey for the German Besetzung, which conveys the idea of something's being filled or occupied. As used in Freud's 1895 "Project for a Scientific Psychology," for example, cathexis refers to the degree to which a neuron is filled with a quantity of energy and hence in a state of altered readiness for discharge.

Freud also makes a crucial (but not fully defensible) assumption that the accumulation of large quantities of cathexis in neurons is a direct source of painful sensations ("unpleasure"), and that "pleasure" consists neurologically in a low level of cathexis. Ucs. material with strong cathexis would break through into consciousness, causing unpleasure, but for an equally strong opposing anti-cathexis. By the time the "structural" theory of the mind is published in 1923, Freud assigns the ego the function of maintaining preconscious "anti-cathexes." These are weakened in sleep, intoxication, and psychosis, allowing the Ucs. material to enter consciousness with little distortion. The ego is also said to direct attention to parts of the Pcs. by "hyper-cathecting" them with attention.