Principal
Principals in JAAS are objects storing user credentials. Principals can then be added (stored) in subjects, which is another object in JAAS to store multiple (or just one) principals. The subject then is propagated with the security context, and is available for the application server to check the logged-in principals.
WebSphere itself has its own implementation of a principal, WSPrincipal. This principal is used internally with the security context.