A cultivar is a plant discovered or developed by a human. The word simply means “cultivated variety” (culti + var). And usually we write the cultivar name in between single quotation marks and in Roman letters. ‘Osiris Fantaisie’, for example, is a cultivar of Ligularia dentata.
Also, to avoid confusion, the rule is that the cultivar name must always be derived from a spoken language such as French, English or Japanese, rather than a dead language like Latin or ancient Greek. The botanical name of the plant, on the contrary, is always written in italics and does derive from ancient Latin or Greek. There are still a few old cultivars with Latin-based cultivar names (‘Rubra’, ‘Variegata’, etc.), but these are exceptions rather than the rule.
Take the name Vitis vinifera ‘Petit Rouge’, a grape vine, as an example. Vitis vinifera, italicized, is the botanical name for all European grapes. ‘Petit Rouge’, between single quotation marks and in Roman letters, is the cultivar name. This name helps distinguish it from the hundreds of other cultivars of European grape.
Note also that you never translate a cultivar name: it’s like a proper name. Frenchman François Roy doesn’t become Francis King when he travels to England and Francis König while he’s in Germany: it remains the same wherever he goes, all over the world. So it would be incorrect to change the name of ‘Petit Rouge’ to ‘Little Red’ when you grow it an English-speaking country.