1660s, "painting or drawing done in different tints of a single color," from Latinized form of Greek monokhrōmos, also monokhrōmatos, "of a single color," from monos "single, alone" (from PIE root *men- (4) "small, isolated") + khrōma (genitive khrōmatos) "color, complexion, skin" (see chroma). As an adjective from 1849. Photographic sense is recorded from 1940.

1959 as a shortening of monophonic in reference to recordings; earlier used among printers for "monotype machine" (c. 1925) and generally for monochrome (motorcar, etc.), 1940s. From 1964 as short for mononucleosis.

"of one color, consisting of light of one wavelength," 1807, from mono- + chromatic, or from monochrome. Perhaps based on French monochromatique or Greek monokhrōmatos "of one color." Related: Monochromatically (1784).

Proto-Indo-European root meaning "small, isolated."

It forms all or part of: malmsey; manometer; monad; monarchy; monastery; monism; monist; monk; mono; mono-; monoceros; monochrome; monocle; monocular; monogamy; monogram; monolith; monologue; monomania; Monophysite; monopoly; monosyllable; monotony.

It is the hypothetical source of/evidence for its existence is provided by: Greek monos "single, alone," manos "rare, sparse;" Armenian manr "thin, slender, small."

c. 1400, separacioun, "a severing, detaching, cutting apart, act of removing or disconnecting one thing from another," from Old French separacion (Modern French séparation) and directly 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"). Alternative separateness (1650s) tends to hold to the meaning "distinct character or state, fact of being separate."

The specific sense of "sundering of a married couple, limited divorce" (without dissolution of the marriage tie) is attested from c. 1600. Sense in printing in reference to proportionate monochrome representations of a color photograph   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). In psychology, the child's separation anxiety is attested from 1943.