ex·pert 
(ĕkspûrt′)
(ĕkspûrt′)n.
A person with a high degree of skill in or knowledge of a certain subject.
adj. (ĕkspûrt, ĭk-spûrt)
Having, involving, or demonstrating skill in or knowledge of a certain subject. See Synonyms at proficient.
[Middle English, from Old French, experienced, from Latin expertus, past participle of experīrī, to try; see per-3 in the Appendix of Indo-European roots.]
expert′ly adv.
expert′ness n.