A statement of what a thing is, drawn tightly enough to exclude what it is not. To define is to set a boundary around a meaning.
Latin definitio (a boundary, definition), from definire (to bound, determine); a statement of what a thing is, drawn tightly enough that what it is not falls plainly outside it.
Latin finis is an ending, and a definition inherits that: it draws the line where a word's meaning stops, not only where it starts. A boundary that cannot say what falls outside it has not defined anything yet.