Fat. Also known as lipid, a category of compounds which are widely distributed in nature. Chemically, they are esters (q.v.) of glycerol (q.v.), a trihydric alcohol (q.v.), and of fatty acids (q.v.), such as stearic, palmitic, oleic, etc. Most fats are triglycerides (q.v.) and, under normal conditions, contain only traces of derivative monoglycerides (q.v.) and diglycerides (q.v.). Their physical state, i.e., whether liquid at ordinary temperatures, or semisolid or plastic, is governed by the nature of their constituent fatty acids. Fats represent a highly concentrated source of energy, are vital cell constituents in plant seeds and animal tissues, and find extensive use in baking as shortenings (q.v.) in product formulas (q.v.) and as a frying medium. Fat derivatives in the form of mono- and diglycerides are important emulsifiers (q.v.) or surfactants (q.v.).