Cultigen

A cultigen is a plant that is the result of artificial selection by humans. Liberty Hyde Bailey, an American botanist was the first to use the term, in 1918. Bailey noticed that the classification Linné introduced for plants was not useful for classifying plants which came from human cultivation and selection. He called the plants which grow in the wild without human selection indigens. A cultigen was: " ... a domesticated group of which the origin may be unknown... [It has] such characters as to separate it from known indigens, and is probably not represented by any type specimen or exact description". The point is that a cultigen cannot always be placed in the traditional Linnean system of botanical classification.
Bailey later changed his definition to "Plant or group known only in cultivation; presumably originating under domestication; contrast with indigen", which is the definition used above. Examples of cultigens are maize and cabbage.
Cultigens and cultivars.
Cultigen and cultivar may be confused with one-another. Cultigen is a general-purpose term encompassing not only plants with cultivar names but others as well (see introductory text above). Cultivar is a formal classification category in the International Code of Nomenclature for Cultivated Plants (ICNCP).
Although in his 1923 paper Bailey used only the rank of species for the cultigen, it was clear to him that many domesticated plants were more like botanical varieties than species and so he established a new classification category for these, the cultivar, generally assumed to be a contraction of the words “cultivated” and “variety”. He defined cultivar in his 1923 paper as: 
 ... "a race subordinate to species, that has originated and persisted under cultivation; it is not necessarily, however, referable to a recognised botanical species. It is essentially the equivalent of the botanical variety except in respect to its origin".
This definition and understanding of cultivar has changed over time.
Definition of cultigen.
The definition has been discussed: