Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
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").