to be or describe the essential quality or nature qualities of (somebody or something); to identify
to set forth the meaning of (e.g. a word)
To define true madness, what is't but to be nothing else but mad? — Shakespeare
to fix or mark the limits of (something); to demarcate
to make (something) clear or precise in outline
definable adj
definer noun
[Middle English definen via French from Latin definire, from de- + finire to limit, end, from finis boundary, end]