Full Idea
A concept is a one-place function - something that can be asserted of an object - as found in 'Earth is a planet' and 'Venus is a planet'. This notion of concept does not belong to psychology at all.
Gist of Idea
A concept is a non-psychological one-place function asserting something of an object
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Joan Weiner - Frege Ch.4
Book Reference
Weiner,Joan: 'Frege' [OUP 1999], p.57
A Reaction
This doesn't seem to leave room for the concept of the object or substance of which the something is asserted. In 'x is a planet' we need a concept of what x is. But then Frege will reduce the reference to a set of descriptions (i.e. functions).