Mono-form
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. () 

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." ()

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." ()

Lifeform classifications