What is a hook-length?

Answer 1

A hook length is quite literally the length of line that the hook is tied to directly. In typical cases, it will be a length of fishing line, usually of a lower breaking strain than the main line, to which the hook is tied at one end, the other end being joined to the main line off the reel.

In other cases, this 'hook length' may be of a different material to the main line, eg. braid, wire etc, which may be in use to perhaps further disguise the hook length, or protect it from teeth etc that may cut it!