Mathematical concepts have been a consensus goal for decades, yet there is little consensus on what is meant by the terms "mathematical concept" and "mathematical understanding." This article attempts to discuss these terms in a way that can be of practical use to teachers. Greater clarity on "mathematical concept" would allow teachers to articulate the concept for themselves (i.e., the logical necessity of the relationship), and they would be in a much stronger position to design or adapt a conceptual lesson and facilitate and assess conceptual learning. Examples of conceptual lessons are provided.