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.
Ponderosa lemon
The ponderosa lemon ("Citrus " x "pyriformis") is a citrus hybrid of a pomelo and a citron. It is not the same as the 'Yuma Ponderosa' lemon pomello hybrid, that is used as citrus rootstock.