Isidore, Etymologiae 6.8:
8. On the Types of Minor Works:
There are three types of little works. The first consists of excerpts, which are called scholia in Greek. In these, those things which seem obscure or difficult are touched upon summarily and in brief. The second type consists of homilies, which the Latins call a verbum, and which are delivered to the people. The third type is tomes, which we call either books or volumes. While homilies are spoken to the crowd, tomes, that is books, are greater disputations. A dialogue is a bringing together of two or more, and the Latins call it a sermo. (For, what the Greeks call dialogos, we call sermones.) The word sermo is employed thus because it is sown (seritur) between each party. Thus, we read in Vergil,
They were sowing many things among themselves.
Sermo however differs from the tractatus [the treatise] and the verbum. For sermo requires another person; the tractatus is self-contained, while the verbum is directed to all. Thus, we have the saying, ‘He delivered a verbum to the populace.’ Commentaries are so called as if they meant cum mente (with mind). For they are interpretations, as the comments upon the law or the Gospel. The apologeticum is a speech of excuse, in which people are accustomed to respond to their accusers, and it rests entirely on either defense or a negation of the accusation; and the term apologeticum is Greek. The panegyricum is a licentious and lascivious type of speaking in honor of kings, in the composition of which people are praised with a heap of lies. This evil arose among the Greeks, whose frivolity puffed up clouds of lies with its skill in speaking and incredible wit. Books of Fasti are those in which either kings or consuls are inscribed, and receive their name from the fasces, that is, from the symbols of power. Thus, Ovid’s books of Fasti are so called because they are derived from kings and consuls. The prooemium is the beginning of speaking, for prooemia are the beginnings of books, which are fashioned at the beginning of a work to prepare the ears of the audience. Many people with experience of Latin simply use this name without translation. But this word, interpreted in our language, is called a praefatio, as if to signify a speaking beforehand. Praecepta are those things which teach us what is to be done or not to be done. In the case of what is to be done, we have ‘Reverence the Lord your God,’ and, ‘Honor your father and mother.’ In the case of what is not to be done, we have ‘Don’t commit adultery,’ and, ‘Don’t steal.’ Similarly, there are gentile precepts which either order or prohibit. There are orders, such as,
Plow naked, and naked sow.
There are prohibitions, such as,
Don’t sow the hazel among the vines, and don’t go for the top of the shoot.
VIII. DE GENERIBVS OPVSCVLORVM. Opusculorum, genera esse tria. Primum genus excerpta sunt, quae Graece scholia nuncupantur; in quibus ea quae videntur obscura vel difficilia summatim ac breviter praestringuntur. Secundum genus homiliae sunt, quas Latini verbum appellant, quae proferuntur in populis. Tertium tomi, quos nos libros vel volumina nuncupamus. Homiliae autem ad vulgus loquuntur, tomi vero, id est libri, maiores sunt disputationes. Dialogus est conlatio duorum vel plurimorum, quem Latini sermonem dicunt. Nam quos Graeci dialogos vocant, nos sermones vocamus. Sermo autem dictus quia inter utrumque seritur. Vnde in Vergilio (Aen. 6,160):
Multa inter se serebant.
Tractatus est ***.
Differt autem sermo, tractatus et verbum. Sermo enim alteram eget personam; tractatus specialiter ad se ipsum est; verbum autem ad omnes. Vnde et dicitur: ‘Verbum fecit ad populum.’ Commentaria dicta, quasi cum mente. Sunt enim interpretationes, ut commenta iuris, conmmenta Evangelii. Apologeticum est excusatio, in quo solent quidam accusantibus respondere. In defensione enim aut negatione sola positum est; et est nomen Graecum. Panegyricum est licentiosum et lasciviosum genus dicendi in laudibus regum, in cuius conpositione homines multis mendaciis adulantur. Quod malum a Graecis exortum est, quorum levitas instructa dicendi facultate et copia incredibili multas mendaciorum nebulas suscitavit. Fastorum libri sunt in quibus reges vel consules scribuntur, a fascibus dicti, id est potestatibus. Vnde et Ovidii libri Fastorum dicuntur, quia de regibus et consulibus editi sunt. Prooemium est initium dicendi. Sunt enim prooemia principia librorum, quae ante causae narrationem ad instruendas audientium aures coaptantur. Cuius nomen plerique latinitatis periti sine translatione posuerunt. Hoc autem vocabulum apud nos interpretatum praefatio nuncupatur, quasi praelocutio. Praecepta sunt quae aut quid faciendum aut quid non faciendum docent. Quid faciendum, ut: ‘Dilige [Dominum] Deum tuum,’ et: ‘honora patrem tuum et matrem tuam.’ Quid non faciendum, ut: ‘Non moechaberis,’ ‘Non furtum facies.’ Similiter et gentilium praecepta vel iubent vel vetant. Iubent faciendum, ut (Virg. Georg. 1,299):
Nudus ara, sere nudus.
Vetant, ut (Virg. Georg. 2,299):
Neve inter vites corylum sere, neve flagella summa pete.