Longman Dictionary of Contemporary English
Wiktionary
n. A large mobile medical unit that temporary takes care of casualty before they can be safely transported to more permanent hospital facilities.
WordNet
n. a temporary military hospital near the battle lines
Wikipedia
A field hospital is a small mobile medical unit, or mini hospital, that temporarily takes care of casualties on-site before they can be safely transported to more permanent hospital facilities. This term is used overwhelmingly with reference to military situations, but may also be used in times of disaster. The concept was inherited from the battlefield (such as the Mobile Army Surgical Hospital or MASH), and is now applied in case of disasters or major accidents, as well as with traditional Military medicine.
A field hospital is a medical staff with a mobile medical kit and, often, a wide tent-like shelter (at times an inflatable structure in modern usage) so that it can be readily set up near the source of the casualties. In an urban environment, the field hospital is often established in an easily accessible and highly visible building (such as a restaurant, school, and so on). In case of an airborne structure, the mobile medical kit is often placed in normalized container; the container itself is then used as shelter. A field hospital is generally larger than a temporary aid station but smaller than a permanent military hospital.
International humanitarian law, such as the Geneva Conventions, include prohibitions on attacking doctors, ambulances, hospital ships, or field hospitals buildings displaying a Red Cross, a Red Crescent or other emblem related to the International Red Cross and Red Crescent Movement.