Any crack that is wider at the back than at the front.
This makes placing traditional climbing gear
such as
nuts ,
rocks or
hexes impossible.
The gear just falls out of the crack.
The first type of gear to be able to protect such a crack
was an SLCD, the first such device is called a friend
and was invented by John Bardeen.
Climbing offwidths involves placing yor hand into the crack and making a fist until your hand expands to fill the space. Then you have to pull up on your fist, mmmm
lovley
as no comply mentioned to me if the crack is sufficiently offwidth then indeed you can't jam the crack. I, bring just a simpering european climber have had few expierences with the true glory of the American offwodth crack
(I''m not kidding here, it is well known that due to the big crags in the states , Yosemite namely, abounding in such cracks American climbers have developed the techniques needed to climb them). Sometimes one has to use pure body tension
in a "double gaston" type manouver to stay on the crack.