SESSSIONID , SessionIndentifier
In computer science, a session identifier or session ID is a piece of data that is used in network communications (often over HTTP) to identify a session, a series of related message exchanges. Session identifiers become necessary in cases where the communications infrastructure uses a stateless protocol such as HTTP. For example, a buyer who visits a seller's site wants to collect a number of articles in a virtual shopping cart and then finalize the shopping spree by going to the site's checkout page. This typically involves an ongoing communication where several webpages are requested by the client and sent back to them by the server. In such a situation, it is vital to keep track of the current state of the shopper's cart, and a session ID is one way to achieve that goal.
A session ID is typically granted to a visitor on his first visit to a site and then for example stored in a cookie by the user's web browser application. The browser will then transparently include the cookie data in all subsequent page requests to the seller's site, thus allowing the server to identify the session and send back a page that may include such data as the number of articles the user has previously requested. What makes this different from a user ID is that sessions are typically short-lived (they expire after a preset time of inactivity which may be minutes or hours) and may become invalid after a certain goal has been met (for example, once the buyer has finalized his order, he can not use the same session ID to add more items).
dont know if this is what you want but here http://www.w3.org/TR/WD-session-id