Full Idea
A concept is for me that which can be predicate of a singular judgement-content.
Gist of Idea
A concept is a possible predicate of a singular judgement
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §66 n)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.77
A Reaction
This seems intuitively odd, given that a predicate could (in principle) be of almost infinite complexity, whereas I would be reluctant to call anything a 'concept' if it couldn't be grasped by a single action of a normal conscious mind.