Final answer:
In Kerberos, any machine or client that can be assigned tickets is referred to as a principal, which includes user accounts and services that require authentication.
Step-by-step explanation:
A server or client that Kerberos can assign tickets to is known as a principal. In the context of the Kerberos authentication protocol a principal may refer to any entity that can be authenticated by the Kerberos server; this includes both user accounts and services that require authentication. Any machine or process that Kerberos can assign tickets to qualifies as a principal.
The other terms listed have specific roles within the Kerberos protocol: Key Distribution Center (KDC) is the service that issues tickets for authentication Ticket Granting Server (TGS) is a part of the KDC that issues ticket-granting tickets (TGTs) and a realm is the domain of authority within which the KDC can issue tickets.