An Etymological Dictionary of Astronomy and Astrophysics

1. A combining form meaning “alone, single, one,” as in monochromatic, Monoceros, monotonic.
   

2. In chemistry applied to compounds containing one atom of a particular element (monohydrate).

Etymology (EN): From Gk. mono-, from monos “single, alone,” from PIE base *men- “small, isolated,” also represented by Gk. manos “rare, sparse.”

Etymology (PE): Tak “single, alone”, from Mid.Pers. tak “single, alone,” maybe related to tâk, tâi “unit, piece.”
Yek “one, alone,” from Mid.Pers. êwak (Proto-Iranian *aiua-ka-); O.Pers. aiva- “one, alone;” Av. aēuua- “one, alone;” cf. Skt. éka- “one, alone, single; " Gk. oios “alone, lonely;” L. unus “one;” Ger. ein; E. one.
Yektâ-, from yek, as above, + tâ “fold, plait, ply; piece, part,” also a multiplicative suffix; Mid.Pers. tâg “piece, part.”
Mono-, loan from Gk., as above.