iuramentum

Iuramentum, iuramenti is neuter, second declension, built from iuro on the same pattern that gives documentum from doceo and monumentum from moneo: the -mentum suffix makes the thing that results from the action.

It is not classical. Cicero and Caesar say ius iurandum, a gerundive phrase in which both halves decline, so the genitive is iuris iurandi and the ablative iure iurando, which trips people badly the first time. Iuramentum is medieval, and once you reach charters, oaths of fealty and court records it is everywhere. Treat it as a dating marker: a text full of iuramenta is not the Republic. Italian giuramento and Spanish juramento come down from it directly; English keeps the root only in jury, perjury and abjure.