Sternlight
From MITNA
The sternlight is a single white light that must be displayed on vessels underway between sunset and sunrise. The light shines over an arc of 135 degrees. This light must be visible for a distance of two miles, except for vessels over 50m (164 ft.) in which case it must be visible for three miles.
On sailboats, the required sternlight and sidelights can be replaced by a tricolor light when under sail power.
On vessels less than 12m (39.4 ft.) operating under motor power, the sternlight can be combined with the masthead light in a single all-round white light.
