Concepts and conceptual understanding: what are we talking about?

The words concept and conceptual understanding are two words used frequently by physics educators yet there is no satisfactory definition of either in the literature. In this article I discuss my own ideas of what these phrases imply for the practice of physics and physics education. I suggest that the word ‘conceptual’ is commonly used to imply qualitative reasoning. Although this seems to involve the use of simple relationships this kind of reasoning is actually far from simple. It requires the coordination of seemingly disparate areas of knowledge and I argue that it is not always easy to distinguish between a misconception or a false concept and a failure to reason correctly.