An expert is someone widely recognized as a reliable source of knowledge, technique, or skill whose judgement is accorded authority and status by the public or their peers. Experts have prolonged or intense experience through practice and education in a particular field. The opposite of an expert is generally known as a layperson, while someone who occupies a middle grade of understanding is generally known as a technician and often employed to assist experts. The concepts of experts and expertise is debated within the field of epistemology under the general heading of expert knowledge.

In specific fields, the definition of expert is well established by consensus and therefore it is not necessary for an individual to have a professional or academic qualification for them to be accepted as an expert. In this respect, a shepherd with 50 years of experience tending flocks would be widely recognized as having complete expertise in the use and training of sheep dogs and the care of sheep. Another example from computer science is that an expert system may be taught by a human and thereafter considered an expert often outperforming human beings at particular tasks. In law, an expert witness must be recognized by argument and authority.

The term is widely used informally, with people being described as 'experts' in order to bolster the relative value of their opinion, when no objective criteria for their expertise is available. The term crank is likewise used to disparage opinions. Academic elitism arises when 'experts' become convinced that only they understand their field of study.