What is Agency?

Legal Definition
Agency law is concerned with any "principal"-"agent" relationship; a relationship in which one person has legal authority to act for another. Such relationships arise from explicit appointment, or by implication. The relationships generally associated with agency law include guardian-ward, executor or administrator-decedent, and employer-employee.

The law of agency is based on the Latin maxim "Qui facit per alium, facit per se," which means "he who acts through another is deemed in law to do it himself." Agency, in its legal sense, nearly always relates to commercial or contractual dealings.
Legal Definition
Contracts. An agreement, express , or implied, by which one of the parties, called the principal, confides to the other, denominated the agent, the management of some business; to be transacted in his name, or on his account, and by which the agent assumes to do the business and to render an account of it. As a general rule, whatever a man do by himself, except in virtue of a delegated authority, he may do by an agent. Combee's Case, 9 Co. 75. Hence the maxim qui facit per alium facit per se.

2. When the agency express, it is created either by deed, or in writing not by deed, or verbally without writing. 3 Chit. Com. Law 104; 9 Ves. 250; 11 Mass. Rep. 27; Ib. 97, 288; 1 Binn. R. 450. When the agency is not express, it may be inferred from the relation of the parties and the nature of the employment, without any proof of any express appointment. 1 Wash. R. 19; 16 East, R. 400; 5 Day's R. 556.

3. The agency must be antecedently given, or subsequently adopted; and in the latter case there must be an act of recognition, or an acquiescence in the act of the agent, from which a recognition may be fairly implied. 9 Cranch, 153, 161; 26 Wend. 193, 226; 6 Man. & Gr. 236, 242; 1 Hare & Wall. Sel. Dec. 420; 2 Kent, Com. 478; Paley on Agency; Livermore on Agency.

4. An agency may be dissolved in two ways – 1, by the act of the principal or the agent; 2, by operation of law.

5. – 1. The agency may be dissolved by the act of one of the parties. 1st. As a general rule, it may be laid down that the principal has a right to revoke the powers which he has given; but this is subject to some exception, of which the following are examples. When the principal has expressly stipulated that the authority shall be irrevocable, and the agent has an interest in its execution; it is to be observed, however, that although there may be an express agreement not to revoke, yet if the agent has no interest in its execution, and there is no consideration for the agreement, it will be considered a nude pact, and the authority may be revoked. But when an authority or power is coupled with an interest, or when it is given for a valuable consideration, or when it is a part of a security, then, unless there is an express stipulation that it shall be revocable, it cannot be revoked, whether it be expressed on the face of the instrument giving the authority, that it be so, or not. Story on Ag. 477; Smith on Merc. L. 71; 2 Liv. on Ag. 308; Paley on Ag. by Lloyd, 184; 3 Chit. Com. f. 223; 2 Mason's R. 244; Id. 342; 8 Wheat. R. 170; 1 Pet. R. 1; 2 Kent, Com. 643, 3d edit.; Story on Bailm. 209; 2 Esp. R. 665; 3 Barnw. & Cressw. 842; 10 Barnw. & Cressw. 731; 2 Story, Eq. Jur. 1041, 1042, 1043

6. – 2. The ageacy may be determined by the renunciation of the agent. If the renunciation be made after it has been partly executed, the agent by renouncing it, becomes liable for the damages which may thereby be sustained by his principal. Story on Ag. 478; Story on Bailm. 436; Jones on Bailm. 101; 4 John r. 84.

7. – 2 The agency is revoked by operation of law in the following cases: 1st. When the agency terminates by the expiration of the period, during which it was to exist, and to have effect; as, if an agency be created to endure a year, or till the happening of a contingency, it becomes extinct at the end or on the happening of the contingency.

8. – 2. When a change of condition, or of state, produces an incapacity in either party; as, if the principal, being a woman, marry, this would be a revocation, because the power of creating an agent is founded on the right of the principal to do the business himself, and a married woman has no such power. For the same reason, when the principal becomes insane, the agency is ipso facto revoked. 8 Wheat. R. 174, 201 to @04; Story on Ag. 481; Story on Bailm. 206. 2 Liv. on Ag. 307. The incapacity of the agent also amounts to a revocation in law, as in case of insanity, and the like, which renders an agent altogether incompetent, but the rule does not reciprocally apply in its full extent. For instance, an infant or a married woman may in some cases be agents, althouah they cannot act for themselves. Co. Litt. 52a.

9. – 3. The death of either principal or agent revokes the agency, unless in cases where the agent has an interest in the thing actually vested in the agent. 8 Wheat. R. 174; Story on Ag. 486 to 499; 2 Greenl. R. 14, 18; but see 4 W. & S. 282; 1 Hare & Wall. Sel. Dec. 415.

10. – 4. The agency is revoked in law, by the extinction of the subject-matter of the agency, or of the principal's power over it, or by the complete execution of the trust. Story on Bailm. 207, Vide generally, 1 Hare & Wall. Sel. Dec. 384, 422; Pal. on Ag.; Story on Ag.; Liv. on Ag.; 2 Bouv. Inst. n. 1269-1382.
-- Bouviers Law Dictionary
Legal Definition
A relation, created either by express or implied contract or by law, whereby one party (called the principal or constituent) delegates the transaction of some lawful business or the authority to do certain acts for him or in relation to his rights or property, with more or less discretionary power, to another person (called the agent, attorney, proxy or delegate) who undertakes to manage the affair and render him an account thereof. State v. Hubbard, 58 Kan. 707, 51 Pac. 290, 39 In R. A. 860; Sternaman v. Insurance Co., 170 N. Y. 13, 62 N. E. 763, 57 In R. A. 318, 88 Am. St. Rep. 625; Wynegar v. State, 157 Ind. 577, 62 N. E. 38. The contract of agency may be defined to be a contract by which one of the contracting parties confides the management of some affair, to be transacted on his account, to the-other party, who undertakes to do the business and render an account of it. 1 Liverm. Prin. & Ag. 2. A contract by which one person, with greater or less discretionary power, undertakes to represent another in certain business relations. Whart. Ag. 1. A relation between two or more persons, by which one party, usually called the' agent or attorney, is authorized to do certain acts foe, or in relation to the rights or property of the other, who is denominated the principal, constituent, or employer. Bouvier.
See also
-- Black's Law Dictionary
Legal Definition
The relation existing between a principal and his agent. See 46 L. R. A. 334.
-- Ballentine's Law Dictionary