18 18 /eɪˈtin/

adjective

1. () Being one more than seventeen  ( xviii , eighteen , 18 )
   being one more than seventeen
     He celebrated his eighteenth birthday with a big party.
   He celebrated his eighteenth birthday with a big party.
   

noun

1. The cardinal number that is the sum of seventeen and one  ( 18 , eighteen , XVIII )
   the cardinal number that is the sum of seventeen and one
    I will turn eighteen years old next month
   I will turn eighteen years old next month