c. 1400, from Old French separacion (Modern French séparation), from Latin separationem (nominative separatio) noun of action from past participle stem of separare "to pull apart," from se- "apart" (see secret (n.)) + parare "make ready, prepare" (from PIE root *pere- (1) "to produce, procure").
Specific sense of "sundering of a married couple" is attested from c. 1600. Sense in photography is from 1922. Separation of powers first recorded 1788, in "Federalist" (Hamilton), from French séparée de la puissance (Montesquieu, 1748). Separation anxiety first attested 1943.