Plain English Meaning
Everything with mass pulls on everything else with mass. The Earth pulls the airplane down, and that pull is what we call weight.
Full Definition
A physical law stating that every mass attracts every other mass with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. In aviation, it is the reason an aircraft has weight: Earth's mass pulls the aircraft's mass downward toward the center of the Earth.
Context Anchor
Seen when studying Newton’s laws and the basic forces acting on an aircraft, especially the downward force of weight.
Derivation
From Latin gravitas, meaning 'heaviness.' 'Universal' means it applies to all matter, everywhere — not just on Earth. Knowing that helps the pilot see weight not as a property of the airplane alone, but as the result of Earth pulling on it.