“A concept is that which is understood by a term, particularly a predicate. To possess a concept is to be able to deploy a term expressing it in making judgements: the ability connects with such things as recognizing when the term applied, and being able to tell the consequences of its application. The term “idea” was formerly used in the same way, but is avoided because of its associations with subjective and mental imagery, which may be irrelevant to the possession of a concept. In the semantics of Frege, a concept is the reference of a predicate, and cannot be referred to by as subjective term.”
Excerpted from: Blackburn, Simon. The Oxford Dictionary of Philosophy. New York: Oxford University Press, 2008.