"Solid" was the Changeling pejorative for monoform species who were not capable of changing their form at will (typically humanoids). The Founders, after constant persecution at the hands of Solids, actively sought to bring "order" to the galaxy by controlling all Solids through the Jem'Hadar and Vorta. (DS9: "The Search, Part II")

The Changelings had the power to turn one of their own into a solid, which was used as a punishment for a severe crime. (DS9: "Broken Link")