Is it ensure, insure, or assure?
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" (cf. Greek manos "rare, sparse," Armenian manr "thin, slender, small," and perhaps English minnow).
mono mon·o (mŏn'ō)
mono- or mon-
One; single; alone: monomorphic.
Monomolecular; monatomic: monolayer.
Containing one atom, molecule, or group: monomer.
A prefix that means "one, only, single," as in monochromatic, having only one color. It is often found in chemical names where it means "containing just one" of the specified atom or group, as in carbon monoxide, which is carbon attached to a single oxygen atom.