A concept is an abstract, universal psychical entity that serves to designate a category or class of entities, events or relations.
A concept is the element of a proposition rather in the way that a word is the element of a sentence. Concepts are abstract in that they omit the differences of the things in their extension, treating them as if they were identical. Concepts are universal in that they apply equally to every thing in their extension.
Concepts are bearers of meaning as opposed to agents of meaning. A single concept can be expressed by any number of languages. The concept 'dog' can be expressed as 'Hund' in German, 'chien' in French, 'perro' in Spanish. The fact that concepts are in some sense language independent makes translation possible; words in various languages "mean the same" because they express one and the same concept.
(This page needs disambiguation, since the word "concept" has at least six more meanings)
See also