Skin is considered an organ because it meets the definition of an organ, which is a group of related cells that combine together to perform one or more specific functions within the body. Skin performs several functions vital to the survival and health of the body, so deserves the label of organ.
Skin is in fact the largest organ on the human body and performs several vital functions for the body. It protects most of the body's other vital organs, as well as its bones, muscles, ligaments and nerves, by acting as an envelope to contain them. Skin thus protects those organs from biological, chemical and mechanical attacks by outside forces. When the body experiences blood loss from a cut or other wound, skin's healing properties help to constrain the blood loss.
Skin also helps to regulate the body's temperature through dissipating heat and sweating to bring the body's temperature back to a normal range. It also protects the body against cold and against excessive amounts of water loss.
In addition, the sense of touch is dependent on the skin. The many nerve endings spread throughout skin cells act to exchange information with the outside world, responding to touch, heat, cold and pain.