A field hospital was a temporary triage area that could be disassembled and moved elsewhere, as opposed to a stationary hospital.
In 2371, Doctor Julian Bashir was tasked with setting up a field hospital on Campor III in a week. When Kira Nerys denied him the use of a runabout to go to Vulcan for medical supplies for this installation, he warned her that inability to do so could throw off the colonization schedule. (DS9: "Defiant")