General relativity

Mass tells spacetime how to curve; curved spacetime tells matter how to move.

Definition

Einstein's 1915 theory in which mass and energy curve the four-dimensional fabric of spacetime, and what we experience as gravity is the motion of objects along that curvature.

Step 1

Predict first

In one or two sentences, what do you think this concept means? Wrong is fine; the small struggle is what makes the answer stick.