However, their meanings differ. "Power" refers to the ability to achieve certain ends, 'authority' refers to the legitimacy, justification and right to exercise that power. For example whilst a mob has the power to punish a criminal, such as through lynching, only the courts have the authority to order capital punishment.

(wikipedia.org)

2. au·thor·i·ty (plural au·thor·i·ties), noun

1. right to command: the right or power to enforce rules or give orders.
2. holder of power: somebody or something with official power.
3. power given to somebody: power to act on behalf of somebody else, or official permission to do something.
4. source of reliable information: a source of reliable information on a subject.
5. administrative body: an official body that is set up by a government to administer an area of activity ( often used in the plural ).
6. justification: a statement that makes somebody believe something is true.
7. quality that is respected: the ability to gain the respect of other people and to influence or control what they do.
8. obvious knowledge and experience: knowledge, skill, or experience worthy of respect.
9. law source of precedent or principle: a law or legal decision that is cited as establishing a precedent or a principle.
10. legitimate power: a form of rule that is seen as legitimate.

(encarta)

3. Synonyms & Antonyms:

  • synonyms:

rule, power, sway. Authority, control, influence denote a power or right to direct the actions or thoughts of others. Authority is a power or right, usually because of rank or office, to issue commands and to punish for violations: to have authority over subordinates. Control is either power or influence applied to the complete and successful direction or manipulation of persons or things: to be in control of a project. Influence is a personal and unofficial power derived from deference of others to one's character, ability, or station; it may be exerted unconsciously or may operate through persuasion: to have influence over one's friends.

  • sovereign, arbiter.

(Dictionary.com Unabridged (v 1.1), based on the Random House Unabridged Dictionary, © Random House, Inc. 2006.)

  • other synonyms:

ascendancy, authorization, beef*, bible, charge, clout*, command, control, credit, domination, dominion, edge, esteem, force, goods*, government, guts*, influence, juice*, jump, jurisdiction, license, mastery, might, permission, permit, pizzazz*, pow*, powerhouse, prerogative, prestige, punch, right, ropes*, rule, say, say so, say-so*, steam, strength, strong arm*, supremacy, sway, upper hand*, warrant, weight, whip hand*, word, zap*

  • antonyms: servility, servitude

( *=informal or slang )

(Source: Roget's New Millennium™ Thesaurus, First Edition (v 1.3.1) Copyright © 2007 by Lexico Publishing Group, LLC. All rights reserved.)