Citron
A source of confusion is that "citron" or similar words in French (and other languages), and English are false friends, as they refer to the lemon. Indeed, into the 16th century, the English name "citron" included the lemon and perhaps the lime as well.
Citrus
The generic name originated from Latin, where it referred to either the plant now known as citron ("C. medica") or a conifer tree ("Thuja"). It is somehow related to the ancient Greek word for cedar, κέδρος ("kédros"). This may be due to perceived similarities in the smell of citrus leaves and fruit with that of cedar. Collectively, "Citrus" fruits and plants are also known by the Romance loanword "agrumes" (literally "sour fruits").