/ku̇-kē/
n. A handle, transaction ID, or other token of agreement between cooperating programs.
"I give him a packet, he gives me back a cookie."
The claim check you get from a dry-cleaning shop is a perfect mundane 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.