person with broad and profound competence in a particular field

An expert is an individual recognized as a reliable source of knowledge or skill in a particular field. An expert’s corresponding ability or expertise is, mostly, based on his/her credential, training, education, profession, publication or experience. Historically, an expert was referred to as a wise old man. Such individual was usually a profound intellectual distinguished for wisdom and sound judgment.[1] In law, an expert witness may contribute specialized knowledge of a field.

Adolf von Becker: The Art Expert
Three biblical wise old men, Cologne Cathedral.

Someone who is not an expert may be called a beginner or a novice if they are new to that field.