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.
Citrumelo
Trifoliate orange, according to Swingle, belongs to a citrus related genera, called Poncirus, while grapefruit equivocally belongs to the genus citrus, hence the botanical name "X citroncirus" is a hybrid genus, derived from citrus and poncirus.