expert

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person with a high degree of skill in or knowledge of a certain subject.
• adjective Having, involving, or demonstrating skill in or knowledge of a certain subject. synonym: proficient.

from The Century Dictionary.

• Having had experience; experienced; practised; trained; taught by use, practice, or experience.
• Skilful; dexterous; adroit; having facility acquired by practice.
• Pertaining to or resulting from experience; due to or proceeding from one having practical knowledge or skill: as, expert workmanship; expert testimony.
• Synonyms Adroit, Dexterous, Expert, etc. (see adroit); trained, practised. See skilful.
• noun An experienced, skilful, or practised person; one skilled or thoroughly informed in any particular department of knowledge or art.
• noun In law, a person who, by virtue of special acquired knowledge or experience on a subject, presumably not within the knowledge of men generally, may testify in a court of justice to matters of opinion thereon, as distinguished from ordinary witnesses, who can in general testify only to facts.
• To experience.
• [⟨ expert, n.] To examine (books, accounts, etc.) as an expert; have examined by an expert: as, the accounts have been experted.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Taught by use, practice, or experience, experienced; having facility of operation or performance from practice; knowing and ready from much practice; clever; skillful
• transitive verb obsolete To experience.
• noun An expert or experienced person; one instructed by experience; one who has skill, experience, or extensive knowledge in his calling or in any special branch of learning.
• noun A specialist in a particular profession or department of science requiring for its mastery peculiar culture and erudition.
• noun A sworn appraiser.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Extraordinarily capable or knowledgeable.
• adjective Characteristic of an expert.
• noun A person with extensive knowledge or ability in a given subject.
• noun chess A player ranking just below master.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person with special knowledge or ability who performs skillfully
• adjective of or relating to or requiring special knowledge to be understood
• adjective having or showing knowledge and skill and aptitude