From LQWiki

A cookie is 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). Syn. magic cookie; see also fortune cookie. Now mainstream in the specific sense of web-browser cookies.

Web-browser cookies, although useful, can sometimes be a privacy risk. Tracking cookies are one of the few forms of spyware that Linux is vulnerable to.

This article is based, in whole or in part, on entry or entries in the Jargon File.