Grease is thick, slippery fat or oil. In kitchens, grease comes from cooking meat like bacon: that slick residue left in the pan is grease. Car engines need grease to keep their metal parts moving smoothly without grinding against each other. Mechanics grease bicycle chains and door hinges to stop them from squeaking and wearing out.

The word can also mean to make something run more smoothly in a figurative sense. When people talk about greasing the wheels, they mean making a process work better or faster, like when a teacher helps grease the wheels of a group project by clarifying confusing instructions.

Grease has a somewhat negative reputation because it's messy and hard to clean. You might complain about greasy hands after working on a bike, or avoid greasy food that leaves oil on your fingers. Still, grease serves essential purposes: without it, engines would seize up, chains would rust, and many machines would stop working.