octonus

From earlier *oktōsnos, equivalent to octō +‎ -nus.

• (Classical Latin) IPA^(key): /okˈtoː.nus/,
• (modern Italianate Ecclesiastical) IPA^(key): /okˈto.nus/,

octōnus (feminine octōna, neuter octōnum); first/second-declension numeral

1. (in the plural) eight each; eight at a time
   • c. 52 BCE, Julius Caesar, Commentarii de Bello Gallico 7.73:

     Huius generis octoni ordines ducti ternos inter se pedes distabant.

     Rows by eights of this kind were dug, they stood apart three feet between them.

   • c. 15 BCE, Vitruvius, De architectura 3.1:

     Pseudodipteros autem sic collocatur, ut in fronte et postico sint columnae octonae, in lateribus cum angularibus quindenae.

     The Pseudodipteros, however, is thus assembled so that there are eight columns each in front and rear, fifteen each on the sides including the corner ones.

   • 1611, Johannes Kepler, Strena seu de nive sexangula 20:

     Etenim si cubo adimas angulos suos octonos resectos lateribus aequalibus, introrsumque componas plane constitues octaedron.

     Moreover if you remove from a cube its eight corners each cut on equal sides, and arrange them facing inwards, you will clearly produce an octahedron.

2. (poetic) eight, eighth, eightfold
   • c. 310 CE – c. 394 CE, Ausonius, Epigrams 62 De Glaucia inmatura Morte praevento.1:

     Laeta bis octono tibi iam sub consule pubes / cingebat teneras, Glaucia adulte, genas.

     Already until thy twice eighth year, glad youth was wreathing thy tender cheeks, grown Glaucias.

This is part of the Latin series of distributive numerals. These numerals are inflected as first/second-declension adjectives; in Classical Latin, they typically accompany plural nouns (with which they agree in case and gender) and have the following functions:

• to express the sense “ s each/apiece”, as in hominis digiti ternos articulos habent, “a man’s fingers have three joints each” (Pliny the Elder, Natural History 11.244.3).

• to express multiplication after a numeral adverb, as in Gallinaciis enim pullis bis deni dies opus sunt, pavoninis ter noveni "hens' need twice ten days, peahens' thrice nine" (Marcus Terentius Varro, Res Rusticae 3.9.10)

• to express the sense of cardinal numerals when used with pluralia tantum (plural-only nouns) such as castra "camp": for example, "twelve camps" is expressed by duodēna castra (Pliny the Elder, Natural History 7.105.5). Distributive forms are regularly used in this context for the number 2 and for all numbers greater than 4. For 1, plural-only nouns are used with plural inflected forms of the cardinal ūnus (“one”), as in ūnae scālae "one flight of stairs" (rather than with forms of the distributive numeral singulus). For 3 and 4, plural-only nouns are used with the plural inflected forms of trīnus and quadrīnus, as in trīna castra "three camps" (rather than with forms of ternus and quaternus, which tend to be used in distributive function).

These adjectives do not normally occur in the singular. Because of this, many grammars and dictionaries treat them as plural-only words and refer to them using the nominative masculine plural form in -ī, rather than the nominative masculine singular form in -us (which is often unattested in Classical Latin). However, some of these adjectives are attested in the singular in Classical Latin poetry (e.g. Sed neque Centauri fuerunt, nec tempore in ullo / esse queunt duplici natura et corpore bino..., Titus Lucretius Carus, De Rerum Natura 5.879, and Sic tu bis fueris consul, bis consul et ille, / inque domo binus conspicietur honor, Publius Ovidius Naso, Epistulae ex Ponto 4.9.64; "corpore bino" here seems to have the sense of "twofold body", and "binus ... honor" the sense of "double/dual/twofold honor"). Singular forms are also attested in postclassical Latin, where these adjectives sometimes have non-distributive meanings (taking an ordinal, cardinal, or collective sense instead). These alternative senses are sometimes continued by Romance descendants (e.g. Spanish noveno (“ninth”) from Latin novēnus).

The genitive plural of singulus is usually singulōrum/singulārum, but distributive numerals greater than one commonly use short genitive plural forms ending in -um rather than the longer forms ending in -ōrum and -ārum.

First/second-declension adjective (distributive, normally plural-only; short genitive plurals in -num preferred).

• octōnārius

• “octoni”, in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
• “octoni”, in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
• octoni in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette.