What do we call an individual who has acquired significant knowledge through training in their field?

The term that best describes an individual who has acquired significant knowledge through training in their field is "Professional." A professional typically undergoes extensive education and training, gaining expertise in a specific area, and often adheres to a code of ethics related to their work. This designation generally implies competence and the ability to perform specialized tasks within a profession.

While an expert is also knowledgeable and skilled, the term often emphasizes a deeper level of mastery in a particular niche, which may go beyond formal training. An amateur, on the other hand, usually refers to someone who engages in a field for enjoyment rather than as a career, suggesting less formal training and expertise. A novice is defined as someone who is new to a field or subject area and lacks the training or experience that comes with being a professional. Therefore, "Professional" correctly captures the essence of significant knowledge acquired through training.