A lubricant is something slippery — usually oil or grease — that you put between two surfaces so they can rub without sticking or wearing out. Engines, bicycle chains, door locks and factory machines all need it. Without lubricant, metal screams and parts die young. Mechanics often just say oil or grease; lubricant is the slightly more careful, technical word. Though actually, people also stretch it: a “social lubricant” is something (often alcohol) that makes talking easier, as if conversation were a machine that needed oil. The short form lube is informal, and that short form also has a sexual use, so the full word is safer when you mean engines. Cooking oil is not a machine lubricant, even though it is oily.