separatrix (n.)

early 15c., separaten, transitive, "remove, detach completely; divide (something), sever the connection or association of," from Latin separatus, past participle of separare "to pull apart," from se- "apart" (see secret (n.)) + parare "make ready, prepare" (from PIE root *pere- (1) "to produce, procure"). Sever (q.v.) is a doublet, via French. Intransitive sense of "to part, be or become disunited or disconnected" is by 1630s of things, 1680s of persons. Related: Separated; separating.