The Collaborative International Dictionary of English v.0.48:

balkline \balkline\ n.
   1. line across a billiard table behind which the cue balls
      are placed at the start of a game.

   Syn: baulk line, string line.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

balkline
    n 1: line across a billiard table behind which the cue balls are
         placed at the start of a game [syn: balkline, baulk-
         line, string line]