Just what is a cultivar? It is short for cultivated variety. But that tells a small part of the story. Some people assume that if something is a cultivar it is a hybrid. Cultivars can be hybrids but they can also be species. Many species show a great deal of variation within their populations. Some of these variations are more attractive as garden plants than others.
Cultivars are often refered to as named varieties. Through the ages special plants whether species or hybrids were often given garden names.
In 1920 when the American Iris Society (AIS) was founded, there were many Irises with cultivar names some of which overlaped. One person's 'King of the Blues' might not be the same as someone else's 'King of the Blues'. Just as bad sometimes the same plant had dozens of different names. It was very hard to order a particular plant and be sure of what would be sent.
The first project of the AIS was to try to sort out the mess and create names that were registered and unique to a particular clone. By the 1950's many plant groups were doing the same, and congresses of horticulturalist met to make some rules for the naming of cultivars. Every four years thousands of horticulturalists come together to create and revise these rules. They are published as the International Code of Nomenclature for Cultivated Plants.
One basic rule the world community follows is that a cultivar name is always written in title case surrounded by single quotes, example 'King of the Blues'. As the code grew many rules were created and 'King of the Blues' would no longer be acceptable as a new name because it claims to be the best, greatest, king and this is an extraordinary claim. Many of the oldest cultivar names violate the present rules but have been grandfathered in. The AIS is the international registrar for all Irises except the bulbous Iris which are registered by the Dutch.