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citron (n.)

"large, thick-rinded, lemon-like citrus fruit," late 14c., also citrine (early 15c.), from Old French citron "citron, lemon" (14c.), possibly from Old Provençal citron, from Latin citrus "citron-tree," and influenced by lemon; or else from augmentative of Latin citreum (malum) "citron (apple);" see citrus.

Apparently the citron was the only citrus fruit known to the Greeks and Romans; the word was used in English to also mean "lemon" or "lime" until the sense became restricted 17c.