An expert is someone who knows a ton about the subject at hand. The adjective form of expert describes someone or something that has that special knowledge. If you get expert instruction, that … EXPERT definition: 1. a person with a high level of knowledge or skill relating to a particular subject or activity…. EXPERT definition: a person who has special skill or knowledge in some particular field; specialist; authority. See examples of expert used in a sentence. If you say that someone has expert hands or an expert eye, you mean that they are very skilful or experienced in using their hands or eyes for a particular purpose. possessing special skill or knowledge. pertaining to or characteristic of an expert: expert advice. Find 132 different ways to say EXPERT, along with antonyms, related words, and example sentences at Thesaurus.com.