Citron
The citron ("Citrus medica") is a large fragrant citrus fruit with a thick rind. It is one of the original citrus fruits from which all other citrus types developed through natural hybrid speciation or artificial hybridization. Though citron cultivars take on a wide variety of physical forms, they are all closely related genetically. It is used widely in Asian cuisine, and also in traditional medicines, perfume, and for religious rituals and offerings. Hybrids of citrons with other citrus are commercially prominent, notably lemons and many limes.
Zest (ingredient)
The zest and mesocarp vary with the genetics of the fruit. Fruit with peels that are almost all flavedo are generally mandarines; relatives of pummelos and citrons tend to have thicker mesocarp. The mesocarp of pummelo relatives (grapefruit, orange, etc.) is generally more bitter; the mesocarp of citron relatives (Mexican and Persian limes, alemows etc.) is milder. The lemon is a hybrid of pummelo, citron, and mandarin. The mesocarp is also edible, and is used to make succade.