A critic (derived from the ancient Greek word "krites" meaning a judge) is a person who offers a value judgement.
The term is used in particular for a professional who regularly judges performances or other work of other people (such as artists, scientists, musicians or actors) and publishes these judgements in a periodical. Such a person is specialized in one field and usually well educated in that field. Professional critics are numerous in the fields of art, music, film (see film critic), theatre, restaurants and scientific publication.
Criticism is the activity performed by a critic. Constructive criticism is the process of offering valid and well-reasoned opinions about the work of others, usually involving both positive and negative comments. Criticism is often a valuable tool in raising and maintaining performance standards.
Although there can be both positive and negative aspects to criticism, the term is often used to emphasize negative comments. A phrase such as "critics of Einstein argue that..." generally refers to people who are sceptical of the value of Einstein's position.