On the Web, a transient cookie, sometimes called a session cookie, is a small file that contains information about a user that disappears when the user's browser is closed. Unlike a persistent cookie, a transient cookie is not stored on your hard drive but is only stored in temporary memory that is erased when the browser is closed.
A transient cookie is created by simply not setting a date in the Set-Cookie option when an application creates the cookie. (For a persistent cookie, an expiration date is set and the cookie is stored on the user's hard drive until the expiration date or until the user deletes it.)
Transient cookies are often used to enable a site to be able to track the pages that a user has visited during a visit so that information can be customized for the user in some way. Some sites use Secure Sockets Layer (SSL) to encrypt the information contained in a cookie.