Terminator
Also called: shadow line.
The line between day and night on the Moon or a planet — where shadows are longest and detail is sharpest.
Along the terminator the Sun is rising or setting for that part of the Moon, so mountains and crater walls cast long shadows that make the relief leap out. A crater that's obvious near the terminator can almost disappear a few days later under a high Sun.
At the eyepiece
Follow the terminator night after night: it moves about 12° across the Moon each day, revealing new craters.