Plain English Meaning
Everything that has mass pulls on everything else that has mass. The bigger the objects, the stronger the pull. The farther apart they are, the weaker the pull. On Earth, this pull is what we feel as gravity holding us and our aircraft down.
Full Definition
The natural law stating that every object with mass attracts every other object with mass, with the strength of the attraction increasing as the masses get larger and decreasing as the distance between them increases. On Earth, this law is what produces weight and what makes objects fall toward the ground when unsupported.
Context Anchor
Encountered in basic aerodynamics when explaining why an aircraft has weight and why lift is needed in flight.
Derivation
From Latin universalis (applying to all) and gravitas (heaviness, weight). The name reflects Newton's insight that the same pulling an apple to the ground also holds the Moon in orbit -- one rule, applying everywhere, to everything.