(covers information from several alternate timelines)
Triage is a medical term for assessing a patient's condition and to prioritize treatment.
In 2368, Doctor Beverly Crusher prepared triages for the wounded crewmembers and survivors of the USS Denver. She built emergency infirmaries in three cargo bays and built teams with all personnel who had a medical background including Toby Russell. (TNG: "Ethics")
In 2372 Neelix interviewed The Doctor for his A Briefing with Neelix program, The Doctor began his statement with the tasks he had to perform when he received patients, the first of which was triage. (VOY: "Investigations")
Soon after, The Doctor was forced to open triage facilities in the USS Voyager sickbay and Holodeck 2 to accommodate all of the injured crewmembers from the effects of proton bursts. (VOY: "Deadlock")
In 2374 in the alternate timeline known as the Year of Hell, The Doctor opened a triage facility in the mess hall after heavy attacks by Krenim warships. Later, when Lieutenant Tom Paris continued to spend time with Lieutenant B'Elanna Torres despite her no longer being in critical danger, The Doctor reminded him that the first rule of medical triage was to make sure that the patient would live, then move on to the next one, emotional detachment being essential. (VOY: "Year of Hell")
That same year, after the USS Voyager was taken over by the Hirogen, The Doctor was faced with a high incidence of casualties and a near impossible level of triage. When he was later ordered to treat Hirogen wounded over Voyager personnel, his argument that the first rule of triage states that the critical injuries take priority was overruled. (VOY: "The Killing Game", "The Killing Game, Part II")