Terminator
The moving line dividing the lit and unlit hemispheres of a body.
The terminator is where the Sun sits on the local horizon — sunrise along the advancing edge, sunset along the retreating one. On the Moon it sweeps across the surface at about 15 km/h at the equator.
It is the most rewarding place to observe. Sunlight arriving at a grazing angle throws long shadows from crater rims and mountain peaks, so relief that is invisible under a high Sun stands out sharply. A full Moon, lit head-on with no terminator on the near side, is the flattest and least interesting view of the month.