A principal is the identity a system uses when deciding who is allowed to act. In security, that identity may be a human user, a service account, an application, or another workload. The principal is the subject of the access decision: policy checks its role, token, attributes, and current state before allowing an action.
Principals matter because authorization must be tied to the right actor, not just to a message, session, or channel. In automated workflows, a bot may receive a request in chat, but it should execute only under the correct principal and only for the approved operation. Confusing one principal with another can create confused-deputy problems, overbroad privilege use, or accidental data release. Defenses include separating delegated user access from app-only access, binding approval to a specific principal, action, and resource, and revalidating that identity at execution time with strong logging and replay resistance.