679. In some parts of the world there is often a slight fall in tide during the middle of the high water period. The effect is to create a longer period of stand at higher water. This special feature is called a (n) __________.

Double high water accurately describes a high-water period characterized by a slight fall in the middle, resulting in a prolonged period of high water. This phenomenon, distinct from apogean or perigean tides which relate to lunar distance and tidal range, and unlike a bore which is a tidal wave, directly corresponds to the question's description of an extended high-water stand with a mid-period dip.