What does SOL stand for?

Solid

One characteristic of a solid is that it might be hard. In the same way that a solid holds its shape, the atoms inside of a solid are not allowed to move around. The solid atoms and molecules are trapped in their places. A state of matter in which the molecules are touching and possessing rigid shape and is not compressible. A solid is a relatively dense, rigid state of matter, with a definite volume and shape. Molecules in solids are often packed close together in regularly repeating patterns, and vibrate around fixed positions.