What does decagōnum mean in Latin?

decagōnum, a noun, means “decagon”.

What is the genitive of decagōnum?

The genitive singular is decagōnī. decagōnum is neuter, second declension.

In classical Latin, [dɛ.kaˈɡoː.nũː]; in ecclesiastical (church) Latin, [de.kaˈɡɔː.num].

noun · neutersecond declension

decagōnum n (genitive decagōnī)

Classical[dɛ.kaˈɡoː.nũː]Ecclesiastical[de.kaˈɡɔː.num]

Medieval Latindecagon
Tunc id cōnstābit ex medietāte sēmidiametrī et ex latere decagōnī eiusdem circulī, quod sīc probātur.Then id will be composed from half the semidiameter [radius] and from the side of the decagon of the same circle, which is proved in this way.c. 1308–1336, Richard of Wallingford, “Quadripartitum Ricardi Walynforde de Sinibus Demonstratis”, in John David Bond, editor, Isis, volume 5, number 1, published 1923, →JSTOR, page 108:

Borrowed from Ancient Greek δεκάγωνον (dekágōnon).