A chemotype (sometimes chemovar) is a chemically distinct entity in a plant or microorganism, with differences in the composition of the secondary metabolites.
With respect to plant biology, the term "chemotype" was coined by Rolf Santesson and his son Johan in 1968, defined as, "...chemically characterized parts of a population of morphologically indistinguishable individuals.
[2] The terms chemotype and chemovar were originally introduced to the ICNB in a proposed revision to one of the nomenclatural rules dealing with infrasubspecific taxonomic subdivisions at the 1962 meeting of the International Microbiological Congress in Montreal.
While largely indistinguishable in appearance, specimens of T. vulgaris may be assigned to one of seven different chemotypes, depending on whether the dominant component of the essential oil is thymol, carvacrol, linalool, geraniol, sabinene hydrate (thuyanol), α-terpineol, or eucalyptol.
[1] Because chemotypes are defined only by the most abundant secondary metabolite, they may have little practical meaning as a group of organisms sharing the same trait.