Due to the popularity of the series M*A*S*H, colloquial use might refer to any mobile military field hospital.
The Mobile Army Surgical Hospital (MASH) refers to a United States Army medical unit serving as a fully functional hospital in a combat area of operations. The units were first established in August 1945, and were deployed during the Korean War and later conflicts. The U.S. Army decommissioned the last MASH unit on 2006-10-16.
The MASH unit was conceived by Michael E. DeBakey and other surgical consultants as the "mobile auxiliary surgical hospital". It was an alternative to the system of portable surgical hospitals, field hospitals, and general hospitals used during World War II. It was designed to get experienced personnel closer to the front, so that the wounded could be treated sooner and with greater success. Casualties were first treated at the point of injury through buddy aid, then routed through a battalion aid station for emergency stabilizing surgery, and finally routed to the MASH for the most extensive treatment. This proved to be highly successful; it was noted that during the Korean War, a seriously wounded soldier that made it to a MASH unit alive had a 97% chance of survival once he received treatment.
The MASH unit made its way into popular culture through the 1968 novel M*A*S*H by Richard Hooker, the 1970 feature film based on the novel, and the long-running television sitcom (1972–1983) based on the movie. A 1953 film, Battle Circus, also took place at a MASH.
MASH units continued to serve in various conflicts including the Vietnam War. In October 1990 the 5th MASH, 44th Medical Brigade, XVIIIth AirBorne Corps Ft. Bragg North Carolina, deployed to Saudi Arabia and was the first fully functional Army Hospital in country. This unit moved forward 6 times, always as the first up hospital for the region. In March 1991 the 5th MASH was operationally attached to the 24th Infantry Division to provide Foward Surgical Care (often right on the front battle lines) to the Combat Units that attacked the western flank of Iraqi Army. In March, 1991, the 159th MASH of the Louisiana Army National Guard operated in Iraq in support of the 3rd Armored Division during Operation Desert Storm.
In 1997, the last MASH unit in South Korea was decommissioned. A decommissioning ceremony was held in South Korea, which was attended by several members of the cast of the M*A*S*H television series, including Larry Linville (who played Frank Burns), and David Ogden Stiers, (who played Charles Emerson Winchester). MASH units have since been replaced by the U.S. Army's Combat Support Hospitals.
Worldwide, the last MASH unit was decommissioned on 2006-02-16. The 212th MASH—based in Miesau Ammo Depot, Germany—was the first US Army hospital established in Iraq in 2003, supporting coalition forces during Operation Iraqi Freedom. It was the most decorated combat hospital in the US Army, with 28 Battle Streamers on the organizational colors. The 212th MASH's last deployment was to Pakistan to support the 2005 Kashmir earthquake relief operations. Its equipment was donated to Pakistan.

Trivia

Col. Harry A. Ferguson was the ex. officer of the Tokyo Army Hospital and aided in the establishment of the MASH program.
The 212th MASH's unit sign now resides at the Army Medical Department's Museum in San Antonio, TX
Out of necessity, the MASH unit depicted in the television series was considerably smaller than many of the MASH units deployed by the United States in the Korean War. In the series, about four surgeons are depicted as being assigned to the unit, the administrative staff consists of the C.O. and his assistant, and few soldiers were shown to be present. By comparison, the 8076TH Army Unit Mobile Army Surgical Hospital had personnel including twelve Nurses, eighty-nine Enlisted Men, one MSC, one Warrant Officer and ten Medical Officers, and on occasion would handle over 600 casualties in a 24 hour period. [1]