Authentication & Secrets
Session Cookie
Also: session ID, session token
A cookie holding the random ID that tells a site you are already logged in.
In more detail
The ID points to a server-side record of the login. Whoever holds the cookie is treated as the user, so it should be unguessable, short-lived and set with the Secure, HttpOnly and SameSite attributes. Stealing it can bypass the password and second factor.