1. <web> HTTP cookie.
2. <protocol> A handle, transaction ID, or other token of agreement between cooperating programs. "I give him a packet, he gives me back a cookie".
The ticket you get from a dry-cleaning shop is an example of a cookie; the only thing it's useful for is to relate a later transaction to this one (so you get the same clothes back).
Compare magic cookie; see also fortune cookie.
3. <security, jargon> A cracker term for the password list on a multi-user computer.
4. <jargon> An adjective describing a computer that just became toast.