"Solid" was the Changeling pejorative for mono-form 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") It was also possible for an infant Changeling to turn a solid into a Changeling, at least one that had been a Changeling previously, by it absorbing itself into the body of the solid, though the infant Changeling died in the process. (DS9: "The Begotten")