Solid is one of the four fundamental states of matter. A dense state resistant to deformation, this condition exists in various forms of both matter and antimatter.
The Kataans used the solid propellant crystalline emiristol to fuel their Kataan probe. (TNG: "The Inner Light")
The mellitus, a cloud creature from Alpha Majoris I, had a natural state that was gaseous, but when it was at rest, it was solid. (TOS: "Wolf in the Fold")