gladius

Gladius, gladii is second declension masculine: the short stabbing sword of the legions, the gladius Hispaniensis, and the reason a fighter in the arena is a gladiator. Probably a Celtic loan rather than native Latin.

One declension point saves confusion. Second declension nouns in -ius regularly contract the genitive singular -ii to -i, so the corpus form gladi is a genitive, not a scribal slip; the same happens with fili from filius. As for register, gladius is the prose word and ensis the poets'. What actually reached the Romance languages was neither: the later Empire fought with the long spatha, which gives French epee and Spanish espada, while gladius survives only in gladiator and in gladiolus, the little sword named for its leaves.