is defined differently in varying academic fields. Psychology defines an expert using superior performance; sociology considers that expertise is a label given to an individual by a group and is interested in the power held by experts; philosophy stresses the kind of knowledge held by experts but not novices. Informally, an expert is someone widely recognized as a reliable source of technique or skill whose faculty for judging or deciding rightly, justly, or wisely is accorded authority and status by peers or the public in a specific well-distinguished domain. An expert, more generally, is a person with extensive knowledge or ability based on research, experience, or occupation and in a particular area of study. Experts are called in for advice
on their respective subject, but they do not always agree on the particulars of a field of study. An expert can be believed, by virtue of
credential, training, education, profession, publication or experience, to have special knowledge of a subject beyond that of the average person, sufficient that others may officially (and legally) rely upon the individual's opinion. Historically, an expert was referred to as a sage (Sophos). The individual was usually a profound thinker distinguished for wisdom and sound judgment.
Experts have a prolonged or intense experience through practice and education in a particular field. In specific fields, the definition of expert is well established by consensus
and therefore it is not always necessary for individuals 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.
Research in this area attempts to understand the relation between expert knowledge, skills and personal characteristics and exceptional performance. Some researchers have investigated the cognitive
structures and processes of experts. The fundamental aim of this research is to describe what it is that experts know and how they use their knowledge to achieve performance that most people assume requires extreme or extraordinary ability. Studies have investigated the factors that enable experts to be fast and accurate.
In addition to technical and academic definitions of "expert", at least one humorous and irreverent explanation exists. In it, "expert" is separated into two parts based upon pronunciation - "ex" and "spurt". The two segments are then described separately, with "ex" described as a "has-been" and "spurt" described as "a drip under pressure". Therefore, an expert can be described as a has-been who is a drip under pressure.