mors

Mors, mortis is third declension feminine, an i-stem, so the genitive plural is mortium. The stem is mort-; the nominative is mort- plus -s with the t dropped in the spelling, which is why mors and mortis look so unalike.

The family is large and regular: morior, "to die", mortuus, "dead", mortalis, "mortal", immortalis, and the noun mortalitas. Keep mors apart from letum, the high poetic word for death, and from cadaver, the body left behind. The corpus sometimes uses mors for a corpse, but that is a medieval stretch and not classical usage, so do not import it into a translation of Livy. Idiom worth noting: morte multare, "to punish with death", and mortem obire, "to meet one's death". French mort, Spanish muerte, Italian morte; English mortal, mortify, mortgage, which is literally a dead pledge, and, cognate rather than borrowed, murder.