What is Implied Authority?

Legal Definition
Implied Authority of Contract is a legal term. In contract law, it is the implied ability of an individual to make a legally binding contract on behalf of an organization, by way of uniform or interaction with the public on behalf of that organization. When a person is wearing a uniform or nametag bearing the logo or trademark of a business or organization; or if that person is functioning in an obviously authorized capacity on behalf of a business or organization, that person carries an Implied Authority of Contract. Implied Authority is authority that is not express or written into the contract, but which the agent is assumed to have in order to transact the business of insurance for the principal. Implied authority is incidental to express authority since not every single detail of an agent's authority can be spelled out in the written contract.

For example, if you are at a restaurant and your waitress tells you that she can give you a free beverage if you pay for an entree, she has made a contract with you on behalf of the business by which she is employed. It is implied that she is given the authority to do so by the fact that she is the sole employee of the business who has been designated to complete a transaction with you. (Whether or not other employees ultimately get involved in your transaction is irrelevant, because it is expected that ideally she is the only person required to complete your business.)

Continuing the scenario, suppose that a manager were to come to your table and inform you that your waitress has made a mistake; that the "free beverage w/ paid entree" offer has expired and that you will be expected to pay for your beverage. At this point the business is in direct violation of a legally enforceable contract made between you, the client, and their employee.

They may penalize the employee for the mistake, but Implied Authority of Contract places them under a legal obligation to honor the terms of the agreement.

The same principle applies to more complex or extreme legal circumstances.

A design professional’s implied authority gives the designer the means to act in ways that are incidental to the exercise of actual authority. It is authority that isn’t written down, but needs to be understood to make the construction process a success.

Besides Implied Authority, look into Apparent Authority and Express Authority as well. In relation to companies, apparent authority is usually referred to as "ostensible authority".
-These three are known as the Agency Relationship
-- Wikipedia
Legal Definition
An agent's power to act on behalf of a principal, intentionally granted by the principal as a result of the principal's conduct, but without an express agreement. Failure to object after a prior exercise of such power may give rise to implied authority.

See Express authority, Actual authority, Inherent authority, and Apparent authority.
Legal Definition
a term given to the power that an agent has or the authority under certain circumstances.