What is Eighteenth ?
Eighteenth is , 18th adj, (noun) referring to 18 The eighteenth of April or April the eighteenth (April 18th). Today’s the seventeenth, so tomorrow must be the eighteenth. That’s the eighteenth invoice we’ve sent out today. It’s his eighteenth birthday next week. 18-wheeler (noun) a large lorry with 18 wheels
source: Easier English, Student Dictionary Upper Intermediate Level