From the book Rebilius Crusoe by Francis William Newman, the term aurea mala, or golden apples is used to describe oranges. Did any of the Latins have an actual name for this fruit, or was it simply something like fructus aureum? Were any of the modern scientific terms for oranges used, such as citrus aurantium, citrus bergamia risso, poncirus trifoliata, or citrus reticulata, or many even just citrus, to refer to any fruit of the genus? Surely, across the vast Roman Empire that once spread all over Europe and in parts of Africa, with a powerful navy, would have discovered these.
According to Quicherat’s French-English dictionary, you can say
malum aureum or
aurantium for translating
An interesting discussion about the Citrus Aurantium can be found in this book (in French). According to the author, the oranges were called
mala aurea or
mala Hesperidum by the Ancients and
aurentia during the Middle Age.
Quod potui, puero silvestri ex arbore lecta 70
aurea mala decem misi; cras altera mittam.
Eclogues III, 71
Tum canit Hesperidum miratam mala puellam;
Eclogues VI, 61
Or in PHI : mala aurea.
(in Flora virgiliana, seu catalogus plantarum in Virgilii operibus occurentium, A. L. A. Fée, 1823)
(I know the question asks about Classical Latin, but to those who come here just interested in how to say orange in any form of Latin, a few additions might be helpful.)
The Vatican's Lexicon Recentis Latinitas uses malum Sinense (dulce) for (sweet) orange. This is actually the name used in the respective Wikipedia entry too. References in that article using such name date back at least since the 17th century.
Notice that the term Citrus aurantium (mentioned in Luc's answer) seems to refer only to the bitter orange, and not to the sweet orange. See the link.