A concept is an abstract, universal mental 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. A single concept can be expressed by any number of languages. The concept DOG can be expressed by '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.