navis

navis, navis is a third declension feminine noun and an i-stem, which shows up in two places: the genitive plural is navium, not navum, and the accusative plural can be navis as well as naves. The ablative singular is navi or nave, both correct and both common.

Roman ships come in types worth knowing when you read history: navis longa is a warship, long and narrow and built for oars, and navis oneraria is a merchantman, broad and sail-driven. The root gives nauta "sailor", nauticus, navigo "to sail", and naufragium "shipwreck"; Greek naus is the same ancient word, and English "navy", "naval", "navigate" and "nausea" (seasickness, from the Greek) all come out of it. French nef and Spanish nave descend directly.