The Collaborative International Dictionary of English v.0.48:
xviii \xviii\, adj. The Roman number symbolizing the value eighteen. Syn: eighteen, 18. [WordNet 1.5]WordNet (r) 3.0 (2006):
xviii adj 1: being one more than seventeen [syn: eighteen, 18, xviii] n 1: the cardinal number that is the sum of seventeen and one [syn: eighteen, 18, XVIII]