You may sometimes find this term as part of the description of an essential oil. It is used to indicate oils of different chemical composition, even though they are obtained from plants which are botanically identical. Different soil conditions and climate can produce variations in the proportions of esters, alcohols, and other basic constituents of the oil, and slight variations of this kind, particularly from season to season, are quite usual. When the difference between crops of a particular plant grown in different conditions is sufficient to produce some variation in the properties of the oil, and this difference is constant, season after season, the resulting oil may be classed as a chemotype, in order to distinguish it from the 'standard' oil from the same plant.
It is important to realise that a chemotype has not been altered or tampered with in any way. Nothing has been added to or taken away from the natural oil distilled from the plant, and the chemical differences between the chemotype and the standard essential oils are those which arise within the plant itself.
Thyme is a plant which shows quite marked variations of this kind, and some suppliers list three or four chemotypes of this oil. Most of them have the advantage of being gentler in their action than normal Thyme, and less likely to sting the skin. Eucalyptus, Rosemary, and Ti-tree chemotypes have been identified, and doubtless more oils will be classified in this way as techniques for identifying the differences become more widely used.