Gel is the theater term for color filters. These are thin sheets of plastic placed in front of a lighting fixture to color its output. The term originated from the sheets being cast from gelatin; these days sheets are typically polyester or polycarbonate films. Previous to gelatin, colored water or silk was used.

Manufacturers of gel include Rosco, Great American Market (GAM), Lee, and Apollo. Sheets are typically 20"x24", which are then cut down by lighting techs to the appropriate size. The size originates, again, from the gelatin days- a standard baker's sheet is 20x24, which was used to cast the sheets. You can now buy them in single sheets, full width rolls 50 feet long, or in pre-cut rolls 50 feet long.

Gel still has a limited life, especially in saturated colors. The color will fade or even melt, depending upon the energy absorption of the color, and the sheet will have to be replaced. In permanent installations and some theatrical uses, colored glass filters or dichroic filters are being used. The main drawback is additional expense and a more limited selection.