expert

Etymology

From Old French, from Latin expertus.

adj

1. Extraordinarily capable or knowledgeable.
   I am expert at making a simple situation complex.
   My cousin is an expert pianist.
2. Characteristic of an expert.
   This problem requires expert knowledge.
3. (obsolete) Proven, experienced, veteran.

noun

1. A person with extensive knowledge or ability in a given subject.
   If an expert says it can't be done, get another expert. - David Ben-Gurion
   We called in several experts on the subject, but they couldn't reach an agreement.
   Plastics are energy-rich substances, which is why many of them burn so readily. Any organism that could unlock and use that energy would do well in the Anthropocene. Terrestrial bacteria and fungi which can manage this trick are already familiar to experts in the field. 2013-07-20, “Welcome to the plastisphere”, in The Economist, volume 408, number 8845
2. (chess) A player ranking just below master.