ius

Ius, iuris, n., third declension. Neuter, so nominative and accusative are both ius and the plural is iura. Spelled jus in older printed editions, where the consonantal i was given its own letter.

The distinction from lex is the thing to learn. Lex is a particular statute that somebody voted for; ius is law as a standing order of right, and by extension a right that a person holds. So ius civile, ius gentium and ius naturale are bodies of law, while iura in the plural are entitlements. Ius dicere is to declare the law, the magistrate's job, and it gives iurisdictio. The bare ablative iure means "rightly, with good reason". Watch out for the homograph: a second ius, iuris, n. means broth or sauce, entirely unrelated, and it is the same word behind the culinary "jus".