This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
Pronation/Supination (Tom Page email@example.com)
"Over" pronation describes a minor misalignment of the leg's forward swing
that causes the footstrike to be skewed to the inside of the heel.
"Over" supination is the reverse - impact is shifted toward the outside of
the heel. (Jim Horalek)
Pronation and supination describe natural and normal motions of the foot
during the walking or running stride. In a normal stride, the outside
portion of the heal strikes the ground first. The foot pronates to absorbe
shock. That is, it rolls inward. At the end of the stride, the foot
re-supinates -- rolls outward-- on push-off.
What the previous writer (Jim Horalek firstname.lastname@example.org) is defining is
`over pronation', and `over supination'. These are excesses of the normal
motions. Note that over pronation is fairly common and many shoes are
designed to counteract this. Over supination is very rare. Most people who
think they over supinate probably just under pronate. Some people who think
they over pronate may in fact pronate a normal amount, but fail to
re-supinate sufficiently at the end of the stride.