Adjective

1. on a level with the eye of the spectator; said of a picture, as hung in an exhibition of pictures
2. at risk (dependent upon success) in a contest or enterprise

   the survival of the company is on the line with this project

   "I have a stalker, my life is on the line!" - Gabrielle, from Desperate Housewives Season 3 episode 13

Adverb

1. Calling on the telephone

   I have John on the line for you.

2. On one of the lines marked on a playing field or court.

   The ball was on the line but the umpire ruled it out.