Plain English Meaning
Coating moving metal parts with oil or grease so they slide smoothly instead of grinding against each other, while also keeping them cool and clean.
Full Definition
The application of oil or grease between moving parts of an engine or mechanical system to reduce friction, carry away heat, flush contaminants, and protect surfaces from wear and corrosion.
Context Anchor
Encountered in aircraft systems, engine operation, preflight inspections, and maintenance discussions whenever oil level, grease points, or moving parts are mentioned.
Derivation
From the Latin lubricare, meaning 'to make slippery.' The aviation meaning keeps that core idea but adds the jobs of cooling, cleaning, and protecting metal surfaces inside an engine.