communication trench
• (n.) A winding or zigzag trench forming a path or communication from one siegework to another, to a magazine, etc.
Boy'au noun ; plural Boyaux or Boyaus [ French boyau gut, a long and narrow place, and (of trenches) a branch. See Bowel .] (Fort.) A winding or zigzag trench forming a path or communication from one siegework to another, to a magazine, etc. 
A boyau is a winding or zigzag trench forming a path or communication from one siege work to another, for example to a magazine, etc.