Mono-form was a name used by Changelings to denote solids, "who'll never know the joys of the Great Link", or that is, beings unable to shape-shift.
The Female Changeling introduced the term to Kira Nerys in 2371, after explaining the Changelings desire to avoid Solids whenever possible. (DS9: "The Search, Part II")
Later, when Laas encountered and entered the runabout that was transporting Odo and Chief O'Brien, O'Brien held a phaser to Laas, prompting the newly encountered Changeling to request to Odo to "Tell the mono-form to put down its weapon." (DS9: "Chimera")
A comparable term to mono-form was the term non-shape-shifter.
Ambassador Lwaxana Troi, in revealing that she wore fake hair explained to Odo that "even us non-shape-shifters have to change who we are once in a while." (DS9: "The Forsaken")