Full Idea
The classical theory is that a concept has a definitional structure in that it is composed of simpler concepts that express necessary and sufficient conditions for falling under the concept, the stock example being unmarried and a man for 'bachelor'.
Gist of Idea
Classically, concepts give necessary and sufficient conditions for falling under them
Source
E Margolis/S Laurence (Concepts [2009], 2.1)
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.6
A Reaction
This is the background idea to philosophy as analysis, and it makes concepts essentially referential, in that they are defined by their ability to pick things out. There must be some degree of truth in the theory.