Solid

Solid is one of the four fundamental states of matter. A solid can best be characterized as a substance of densely-packed molecules, which have sufficient potential energy to conduct thermal and/or electricity. (See: entropy and Laws of Thermodynamics). Or become kinetic energy and direct forces from one point in space to another. Its atomic and subatomic (quarks) structure are known to also generate the phenomenon known as gravity. Solids generally have fixed volume and shape, but can expand its surface area. Such as ice crystals. Periodic table elements contain the most abundant forms of this matter, throughout the known universe. Its interactive properties on the quantum level has led scientists such as Paul Manheim to correlate it to the curvature of space/time. As well as gravity. (TNG video game: Future's Past, TNG episode: "We'll Always Have Paris")

"Solid" is also a Changeling term for monoform humanoids who are not capable of changing their forms at will. (DS9 episode: "The Search, Part I")

In 2372, the Changeling impersonating Linda Addison noted that Jean-Luc Picard was very good at hiding his feelings, for a solid. (TNG - Slings and Arrows eBook: A Sea of Troubles)