Triage is the process of prioritizing patient treatment based on the severity of their injuries.
Generally triage involved dividing patients into three categories: (a) those who were injured but who did not need immediate assistance; (b) those who could be saved with immediate assistance; and (c) those so severely injured that no amount of medical assistance could help them. (TNG reference: Star Trek: The Next Generation Technical Manual)
After the USS Denver crashed on a planet in the Mericor system, Doctor Beverly Crusher converted the USS Enterprise shuttlebays into triage centers to classify and deal with the large number of wounded from that disaster. (TNG episode: "Ethics")
On the day that the Cardassians left Bajor after the Occupation of Bajor ended, Kira Nerys was being treated in Dahkur City for the the effects of lake flu. (DS9 - Millennium novel: The Fall of Terok Nor)
When the crew of the USS Defiant entered the Badlands searching for the rogue artificial quantum singularity, triage teams were standing by to administer cellular regeneration treatments to combat tetryon radiation. (DS9 - The Badlands novel: The Badlands, Part IV)