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.

• To experience.
• [⟨ expert, n.] To examine (books, accounts, etc.) as an expert; have examined by an expert: as, the accounts have been experted.
• 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.

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