The term 'actual authority' refers to those situations where the principal has in fact granted an agency power to the agent, either expressly (express authority) or impliedly (implied authority).
The term 'ostensible authority' refers to a situation where there is no actual authority, but where the principal has behaved in a way that makes it appear to the third party that the agent has the authority in question. A principal is bound by an agent's ostensible authority just as if it were actual authority.