freeboard

noun

1. (nautical) The vertical distance between the waterline and the uppermost watertight deck of a vessel.

2. The distance between a water level and the top of something that contains or restrains it (such as a dam).

3. The distance between the top of sea ice and the water level.

4. a type of skateboard which simulates the movement of a snowboard when used on a downhill coarse, allowing snowboarding techniques, which has an addition of two centerline casters that extend below the traditional skateboard wheels and bogies.