noun. In fort, a ditch covered with a parapet, serving as a means of communication between two trenches, especially between the first and third parallels. Also called a zigzag or an approach.

noun. A winding or zigzag trench forming a path or communication from one siegework to another, to a magazine, etc.

noun. In military fortification, a trench or ditch covered with a parapet that provides communication between two trenches, particularly the rear and front lines. Boyaus were typically constructed in a zigzag pattern to prevent exposure to direct enemy fire from the front.

noun. A line, drawn winding about, in order to enclose several tracts of land, or to attack some works.

"We have no direct evidence how the embryo is formed, yet no one doubts but that it is brought about by the agency of the boyau, which is a cell containing grumous molecular matter."

entrenchment  natural-looking  on/off  pitter  underpass