10 letters · 17 points in Scrabble
noun position 18 in a countable series of things
adjective coming next after the seventeenth in position
originInherited from Middle English eightetenthe, eightetethe, from Old English eahtatēoþa, from Proto-Germanic *ahtudô tehundô. Equivalent to eighteen + -th (ordinal suffix).
Etymology from Wiktionary, CC BY-SA 3.0