A "citron" is a fruit resembling a lemon with a pale-yellow color. Its larger size and thicker rind distinguish a citron from a lemon. The term "citron" also defines the tree that produces this particular fruit.
The rind of the fruit is also referred to as a "citron." Objects that bear a similar color to the fruit are identified as being of citron color. This term originated in the late 14th to early 15th centuries in Middle English, notes Dictionary.com. The term "citron melon," an Americanism referring to a particular type of melon with a hard flesh and a pale-yellow color, originated in the 19th century.