The mid-point of the rope folded against itself and/or any section of the rope where it forms a loop by being folded against itself.
When tying for shibari, rope is doubled over from the start, so most ties begin with the bight. Because of this, working end and running/standing end do not have different meaning like they do in other knot tying.
In nautical and other knot tying, a bight is a curved section or slack part between the two ends of a rope, string, or yarn. A knot that can be tied using only the bight of a rope, without access to the ends, is described as "in the bight." By this definition all knots in shibari are considered "in the bight."