What is a plant chemotype, anyway?

Many plant species exhibit chemical polymorphisms in the composition of specialized metabolites belonging to certain chemical families. This has led to the classification of chemotypes, that is, groups of plants that can be distinguished by their chemical profiles of metabolites within a single chemical family. We present existing definitions and approaches for classifying chemotypes and describe the factors that determine them. We argue that it should always be stated explicitly on which organ the chemotype specification is based on, because chemical profiles can differ among organs. Moreover, the chemical family must be stated explicitly, as plants may be grouped differently when other metabolites are taken into account. We argue that gaining more knowledge about chemotypes is highly relevant to both basic and applied science.