A semitropical evergreen tree (Citrus medica) and its fruit. Citron was the first citrus to be introduced to Europe from the source of all edible citrus fruits, Asia, way back before the christian era.
The citron fruit is large - 6 to 9 inches/15 to 22 cm long - and lumpy; it has a thin yellowish-green outer rind and a thick albedo (inner rind), as well as sparse, sour, acidic pulp. The juice is sometimes used to make beverages or syrup, but most citron is grown today for its peel, which is processed in brine and pressed to extract citron oil, which is used to flavour liqueurs and in perfumery. Then the peel is candied, which may be how you've seen it; sometimes half citrons are candied too, but strips or chopped is much more common. Candied citron is a common addition to Christmas fruit cake. Store candied citron in the freezer for maximum freshness.
In French, citron means lemon (citron vert is lime); the stress is on the second syllable: see-TRAWN.