The center part of a length of rope, string, or yarn as opposed to the ends.
A "bight" is any curved section, slack part, or loop between the ends of a rope.
The phrase "in the bight" implies a U-shaped section of rope is itself being used in making a knot. Many knots can be tied either with the end or in the bight.
More a ropeworker's term than a knot term, the reference is to the end of a rope that is tied off, hence the expression "to the bitter end". A bitt is a metal block with a crosspin used for tying lines to, found on docks.
A full circle formed by passing the working end over itself. Note that the term 'loop' is also used to refer to a category of knots (see 'Categories' below).