The one hemisphere
of the Moon
that is always facing away
from the earth. The rotation
of the moon matches its revolution
around the Earth
and hence it keeps one side always facing us. This has partly due to the fact that the center of mass
of the Moon is off center and facing towards Earth's gravity. Over time Earth's gravitational
pull slowed down the Moon's rotation to the equilibrium
it is in today.
The far side is also often mistaken to be the dark side of the moon even though it receives equal amounts of sunlight as the Earth side. This is a good reflection of human psychology as well.