A hole with water, such as a stream or lake, that forces the players to shoot over it
The radio domain between the 21-centimeter wavelength spectral line of hydrogen (H) and the 18-centimeter wavelength spectral lines of the hydroxyl molecule (OH).
A hole that has a large, conspicuous water hazard between the teeing ground and the putting green.
In SETI, the Water Hole is the band of radio frequencies between 1420 MHz and 1640 MHz. This is the band between the hydrogen line and the strongest hydroxyl spectral line, and is notable because it is a quiet region between two notable frequencies; it is theorized that this would be a good band for communications to or from extraterrestrial intelligences.