"solid part of a battlement," between and separating two crenelles or embrasures, 1704, from French merlon (17c.), from Italian merlone, augmentative of merlo "battlement," perhaps [OED] a contraction of mergola, diminutive of Latin mergae "two-pronged pitchfork."
"having the power or tendency of separating," 1580s, from Medieval Latin segregativus, from Latin segregare "set apart, lay aside; isolate; divide" (see segregate (v.)).
1580s, "operation of cutting open or separating into parts," from French dissection (16c.) or directly from Medieval Latin dissectionem (nominative dissectio), noun of action from past-participle stem of Latin dissecare "cut in pieces," from dis- "apart" (see dis-) + secare "to cut" (from PIE root *sek- "to cut"). Meaning "process of cutting open an animal or plant for examination of organs and tissues" is from c. 1600. Transferred sense of "act of separating anything into distinct parts for critical examination" is from 1640s.
1887, "a centrifuge machine," originally a machine for separating cream from milk, from French centrifuge, from noun use of adjective meaning "centrifugal" (1801), from Modern Latin centrifugus (see centrifugal). Centrifuge machine is from 1765.
"wall separating two cavities," especially "the partition between the nostrils," 1690s, Modern Latin, from Latin saeptum "a fence, enclosure, partition," from neuter past participle of saepire "to hedge in," from saepes"a hedge, a fence," which de Vaan suggests is from a PIE *seh-i- "to tie." Related: Septal.
c. 1600, "separatist, one who separates," agent noun in Latin form from separate (v.) or from Late Latin separator "one who separates." As a mechanical device for separating one thing from another, by 1831.
mountain range between the Black and Caspian seas, separating Europe and the Middle East, from Latin Caucasus, from Greek kaukasis, said by Pliny ("Natural History," book six, chap. XVII) to be from a Scythian word similar to kroy-khasis, literally "(the mountain) ice-shining, white with snow." But possibly from a Pelasgian root *kau- meaning "mountain."
1640s, "purification of the mind or soul" (figurative); 1650s, "act or process of separating from lees or dregs, a cleansing from impurities," from Late Latin defecationem (nominative deficatio), noun of action from past-participle stem of Latin defaecare "cleanse from dregs, purify," from the phrase de faece "from dregs" (see de- + feces). Meaning "act of evacuating the bowels" is from 1830. An Old English word for "bowel movement" was arse-gang literally "arse-going."
"line separating waters flowing into different rivers," 1803, from water (n.1) + shed in a topographical sense of "ridge of high ground between two valleys or lower ground, a divide," for which see shed (n.2). Perhaps a loan-translation of German Wasser-scheide. Figurative sense is attested from 1878. Meaning "ground of a river system" is from 1878.