we will decide at this point to call this arrest in a component impulse at an early-stage a FIXATION (of the impulse). The second danger in a development by stages such as this we call REGRESSION
Freud provides his canonical definitional statement of fixation as the arrest of a component instinct at an early developmental stage, paired immediately with regression as its dynamic complement.
, Introductory Lectures on Psycho-Analysis, 1917thesis