Any death where the victim's body is rendered uninhabitable due to extreme stresses. Typically the crushing is performed by heavy machinery, but it could also be achieved through very strong squeezing (eg: Bolo in Enter the Dragon crushes an unnamed guard's skull with his bare hands).
We also class "Compressive Asphyxia" as being crushed. This is where compression of the chest of abdomen causes an inability to breathe. For example, if a car falls off its supports onto the mechanic working beneath it, death can often be caused by asphyxia rather than internal injuries.
There are also various wrestling / grappling moves designed to attack the body in this way, for example placing a knee on the stomach and then raising up so all their weight is put on it.
Compare with Crushed by Falling Object