The Collaborative International Dictionary of English v.0.48:
Citron \Cit"ron\ (s[i^]t"r[u^]n), n. [F. citron, LL. citro, fr.
L. citrus citron tree (cf. citreum, sc. malum, a citron),
from Gr. ki`tron citron]
1. (Bot) A fruit resembling a lemon, but larger, and
pleasantly aromatic; it is produced by the citron tree
(Citrus medica). The thick rind, when candied, is the
citron of commerce. The fruit was once called the lime.
2. A citron tree, Citrus medica.
3. A citron melon.
(a) A small variety of muskmelon with sugary greenish
(b) A small variety of watermelon, whose solid white flesh
is used in making sweetmeats and preserves.
Citron tree (Bot.), the tree which bears citrons. It was
probably a native of northern India, and is now understood
to be the typical form of Citrus Medica.