Expert

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. In law, an expert witness may contribute specialized knowledge of a field.
Someone who is not an expert may be called a beginner or a novice if they are new to that field.