Longman Dictionary of Contemporary English
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▪ a monosyllabic word
The Collaborative International Dictionary
Mono- \Mon"o-\, Mon- \Mon-\ [Gr. ?.] A prefix signifying one, single, alone; as, monocarp, monopoly; (Chem.) indicating that a compound contains one atom, one radical, or one group of that to the name of which it is united; as, monoxide, monosulphide, monatomic, etc.
Douglas Harper's Etymology Dictionary
word-forming element meaning "one, alone; containing one (atom, etc.)," from Greek mono-, comb. form of monos "single, alone," from PIE root *men- "small, isolated" (cognates: Greek manos "rare, sparse," Armenian manr "thin, slender, small," and perhaps English minnow).