The space between a goaltender's legs or pads. The five hole is an area that is often left uncovered by a goaltender because he is usually standing and moving in his crease, so it becomes an area where players often shoot.
The "five" hole is named that way because it is the fifth "hole" or uncovered area in the net when a goaltender is in his standard position. The other four "holes" in the net are found at the top left & the top right (over or near the goalie's left and right shoulders), and the bottom left & bottom right (low stick-side and glove-side).
A goalie technique used to cover the five hole when a goalie faces a shot is the butterfly style technique. To perform the butterfly technique, the goalie will bring his knees together and fall to the ice on his knees while positioning his goalie stick in front of his knees to completely cover the five hole. This technique was introduced by legendary NHL goalie Glenn Hall and was popularized by another goalie legend, Patrick Roy.