an oil derived from both animals and plants; chemically, a glyceride of a fatty acid that, by substitution of the glycerine by an alkaline base, is converted into a soap; a fatty oil, in contrast to a volatile oil, is permanent, leaving a stain on an absorbent surface, and thus is not capable of distillation; it is obtained by expression or extraction; the consistency varies with the temperature, some being liquid (o.’s proper), others semisolid (fats), and others solid (tallows) at ordinary temperatures; both liquid and semisolid oils are congealed by cold and the solids are liquified by heat. SYN: fixed oil.