Amputation is the removal of a body extremity by trauma or surgery.
Some types of amputation are foot amputations, ankle disarticulation, below-knee amputation (transtibial), knee-bearing amputation (Symes), above knee amputation (transfemoral), hip disarticulation, amputation of digits, metacarpal amputation, wrist disarticulation, forearm amputation, elbow disarticulation, above-elbow amputation (transradial), and shoulder disarticulation and forequarter amputation.
Some amputees experience the phenomenon of phantom limbs; they feel body parts that are no longer there. These limbs can itch, ache, and feel as if they are moving. Some scientists believe it has to do with a kind of neural map that the brain has of the body, which sends information to the brain about limbs regardless of their existence.
Amputation is usually done for medical reasons, but in some countries also as official punishment. A few people have an "amputation fetish," feeling that they are not "complete" until they have a certain part amputated. Psychologists describe the little understood disorder as sexual in origin.
In rare cases one of somebody's limbs gets stuck in a deserted place, the person has no means of communication, and there is nobody to help or to get help, and therefore the victim carries out an amputation on him- or herself: In 2003 27-year old Aron Ralston amputated his forearm using his pocketknife and breaking and tearing the two bones, after the arm got stuck under a boulder when hiking in Utah. A month later, an Australian coal miner amputated his own arm with a Stanley knife after it became trapped when the front-end loader he was driving overturned three kilometers underground. 
Amputation can also be done to stop the spread of gangrene and is the most common treatment for cases that respond to nothing else.