Scholia (singular scholium or scholion, from Ancient Greek: σχόλιον, "comment, interpretation") are grammatical, critical, or explanatory comments, either original or extracted from pre-existing commentaries, which are inserted on the margin of the manuscript of an ancient author, as glosses. One who writes scholia is a scholiast. The earliest attested use of the word dates to the 1st century BC.
Ancient scholia are important sources of information about many aspects of the ancient world, especially ancient literary history. The earliest scholia, usually anonymous, date to the 5th or 4th century BC (such as the "a" scholia on the Iliad). The practice of compiling scholia continued to late Byzantine times, outstanding examples being Archbishop Eustathius' massive commentaries to Homer in the 12th century and the scholia recentiora of Thomas Magister and Demetrius Triclinius in the 14th.
Scholia were altered by successive copyists and owners of the manuscript, and in some cases, increased to such an extent that there was no longer room for them in the margin, and it became necessary to make them into a separate work. At first, they were taken from one commentary only, subsequently from several. This is indicated by the repetition of the lemma ("headword"), or by the use of such phrases as "or thus", "alternatively", "according to some", to introduce different explanations, or by the explicit quotation of different sources.
The most important are those on the Homeric Iliad, especially those found in the 10th-century manuscripts discovered by Villoison in 1781 in the Biblioteca Marciana in Venice (see further Venetus A, Homeric scholarship). The scholia on Hesiod, Pindar, Sophocles, Aristophanes and Apollonius Rhodius are also extremely important. In Latin, the most important are those of Servius on Virgil, of Acro and Porphyrio on Horace, and of Donatus on Terence.
Some ancient scholia are of sufficient quality and importance to be labelled "commentaries" instead. The existence of a commercial translation is often used to distinguish between "scholia" and "commentaries". The following is a chronological list of ancient commentaries written defined as those for which commercial translations have been made: