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 is attested by 1792, from French séparée de la puissance (Montesquieu, 1748). The idea was discussed in several places in "The Federalist" (1788), but not in that exact phrase (e.g. separation of the departments of power, No. 81). Separation anxiety is attested from 1943.