vetus

Vetus, veteris is a third-declension adjective of one termination: one form, vetus, serves all three genders in the nominative singular, and everything after that runs off the stem veter-. It is also one of the few that refuse the i-stem endings, so the ablative singular is vetere, not veteri, and the genitive plural veterum, not veterium. The comparative is vetustior and the superlative veterrimus, both irregular enough to be worth memorising.

The sense is long-standing rather than antique: a vetus amicus is a friend of many years, where antiquus points to a former age. Veteres as a noun means the ancients, the old writers. Vetus Testamentum is the Old Testament. English veteran and inveterate come from it.