/ay"teenth"/ , adj.
1. next after the seventeenth; being the ordinal number for 18.
2. being one of 18 equal parts.
n.
3. an eighteenth part, esp. of one ( 1 / 18 ).
4. the eighteenth member of a series.
[ bef. 900; ME eightenthe, eightethe, OE eahtateotha. See EIGHTEEN + -TH 2 ]