A prisoner of war, or POW, was a person who was captured by an opposing group during a time of conflict. Such prisoners were held in prisoner of war camps.
During the Federation-Klingon War in 2256, Lieutenant Ash Tyler of the USS Yeager was captured following the Battle of the Binary Stars. The Klingons then used Tyler's DNA in a ChoH'a' procedure on VoQ. Following this procedure, VoQ, under the guise of Tyler, believed he was held as a POW for 227 days. (DIS: "Battle at the Binary Stars", "Choose Your Pain", "The War Without, The War Within")
When confronting the USS Enterprise-D in orbit of Nelvana III in 2366, Commander Tomalak commanded the crew of the ship to surrender as prisoners of war. (TNG: "The Defector")
That same year, when Counselor Deanna Troi and Doctor Beverly Crusher detailed the alterations made to Roga Danar to condition him for war, including psychological manipulation and alterations to his cell structure, Commander William T. Riker asked if he had been a prisoner of war. Troi explained, however, that the alterations had been made by the Angosian government to program him to be the perfect soldier. (TNG: "The Hunted")
The Seldonis IV Convention governed the treatment of prisoners of war among its signatories, which included the Federation and the Cardassians. A person could only be deemed a prisoner of war (and get the protections of the treaty) if they were acting on behalf of one of its signatories. (TNG: "Chain Of Command, Part II")
In the early months of the Dominion War in 2374, Captain Benjamin Sisko and his crew crashed on a planet in a Cardassian dark matter nebula. There, they encountered Keevan and several Jem'Hadar troops. The Vorta later betrayed his men and gave Sisko their exact plan of attack. Keevan was willing to spend the rest of the war as a prisoner of war. (DS9: "Rocks and Shoals")
Later that year, several Jem'Hadar soldiers serving under Gelnon captured the USS Defiant. Sisko and his crew were able to regain control of the starship, with help from Dax, O'Brien, and Bashir aboard the shrunken USS Rubicon. The Jem'Hadar were then transferred to a Federation prisoner of war camp. (DS9: "One Little Ship")
Curiously, writer Ronald D. Moore wrote six months after this episode aired that "There are no Jem'Hadar POWs." (AOL chat, 1998)