apud
Latin for 'in the works of' — cites a passage as found in another author's text.
apud means 'at', 'by', or 'in the writings of'. In citation it introduces the author in whose work a cited passage is preserved: 'Chrysippus apud Diogenes Laertius' means the Chrysippus material survives only because Diogenes Laertius quoted it.
It is the technical form of the everyday phrase 'as quoted in', and it is indispensable in classics, patristics, and medieval studies, where a very large share of ancient writing survives solely as quotation inside later authors. Those preserved fragments are the testimonia of a lost work.
Correct practice pairs apud with a locatable reference in the surviving text, not the lost one: cite where the modern reader can actually find the words. If the fragment has a standard fragment number, give both.
Common mistake: using apud when you simply read a source in an anthology. Anthologised reprints are not fragments — cite the original publication, since it still exists and can be checked.
“Zeno of Citium apud Diogenes Laertius, Lives 7.87”
Keep reading the glossary
The honest way to cite a quotation you found in a second-hand source rather than the original.
Collected quotations from later writers that preserve the words or ideas of an earlier lost author.
A later account that reports a quote, rather than recording it directly.
A 'chain' of quoted commentaries on a scripture or text, one author after another.