Nouns

(n)veteran, veteran soldiera serviceman who has seen considerable active service“the veterans laughed at the new recruits”
(n)veteran, veta person who has served in the armed forces
(n)veteran, old-timer, oldtimer, old hand, warhorse, old stager, stageran experienced person who has been through many battles; someone who has given long service

Adjectives

(s)seasoned, veteranrendered competent through trial and experience“a seasoned traveler”, “veteran steadiness”, “a veteran officer”