1. Short for "gelatin", a colloidal system comprising a solid and a liquid phase which exists as a solid or semisolid mass: Dr. Smith suggested that Jenny use a special gel to use in her nose to keep it lubricated.

A gel can be explained as a phase that is largely liquid but incapable of flow because it is held rigid by molecular chains, usually cross-linked, that pass through it.
2. A jelly-like substance used in styling hair: Before going off to school in the morning, Max used some gel on his hair to make it look nice.

A colloid with a solid continuous phase and liquid as the dispersed phase: Gels may be unstable when, due to temperature or other cause, the solid phase liquifies and the resulting colloid is called a sol (bright or luminous like the sun).