To state what a thing is, tightly enough to exclude what it is not. To define is to draw a boundary around a meaning.
Latin definire (to limit, determine, define), from de- (thoroughly) + finire (to bound, end), from finis (boundary); to state what a thing is by drawing a boundary tight enough to exclude what it is not.
Its Latin root, finis, is an ending — to define a thing is to decide, in advance, where it stops being itself. A meaning drawn without that edge has not yet been defined, only described.