Mountain biking slang for a
bike that lacks
rear
suspension. Most
cross country bikes are hardtails, since
suspension tends to make it harder to
pedal fast. The weight of a
cross country bike is
extremely important, so anything that isn't
absolutely necessary is usually
stripped off.
On the other hand, in downhill racing rear suspension is a
must. Weight isn't as important in that discipline, but protecting
your body from shocks is.