a thick fatty oil (especially one used to lubricate machinery)
"The mechanic wiped his hands on a rag after applying fresh grease to the engine bearings."
Animal fat in a melted or soft state
"The old mechanic scooped out a handful of grease from the drain to check its consistency before adding it back into the engine."
In plain English: Grease is an oily, slippery substance that often comes from cooking fat and can make things dirty if it gets on your clothes or skin.
"The mechanic used some grease to lubricate the squeaky hinge on the door."
Usage: As a noun, grease refers specifically to animal fat that has been rendered into a liquid or semi-solid form. It is distinct from general oil because of its origin and thicker consistency when cooled.
To put grease or fat on something, especially in order to lubricate.
"She carefully greased the hinges of the old wooden door to ensure it opened smoothly."
In plain English: To grease something means to put oil on it so that parts can move smoothly without rubbing against each other too hard.
"You should grease your bike chain before winter to keep everything running smoothly."
The word "grease" comes from the Old French term for fat or grease, which itself was borrowed into English via Anglo-Norman and Middle English. Its ultimate origin is a Vulgar Latin form derived from the classical Latin adjective meaning "fat" or "thick."