Unconditional surrender was a surrender in which no guarantees or concessions were given to the surrendering party.
After taking control of the USS Defiant in 2155, Jonathan Archer sent a message to Starfleet demanding its unconditional surrender. Admiral Gardner then contacted Archer and warned him that Starfleet would fire on the Defiant if it approached Earth. (ENT: "In a Mirror, Darkly, Part II")
In 2268, a Klingon captain twice demanded the USS Enterprise to surrender unconditionally and immediately to him and rejected James T. Kirk's request for terms, including a guarantee of safety for the Dohlman of Elas, Elaan. (TOS: "Elaan of Troyius")
In 2364, Ferengi DaiMon Tarr called an unconditional surrender "totally unacceptable", claiming the crew of his marauder would rather die than accept such a dishonor. (TNG: "The Last Outpost")
In 2367, William T. Riker claimed to be willing to end hostilities with the Borg and suggested a meeting to discuss terms. Locutus of Borg rejected this and demanded that the USS Enterprise-D must surrender unconditionally. (TNG: "The Best of Both Worlds, Part II")
In 2369, Gul Jasad demanded the unconditional surrender of Deep Space 9. When Kira Nerys claimed she needed a day to make the necessary preparations, Jasad only granted her one hour. (DS9: "Emissary")
After being captured by the Terran Resistance Forces in 2375, Regent Worf asked his captors for their terms; Miles O'Brien replied that a "simple unconditional surrender" would suffice, which Worf accepted. (DS9: "The Emperor's New Cloak")
In 2377, Harry Kim pretended to offer the surrender of the Nightingale to an Annari warship. The Annari Commander demanded Kim to surrender unconditionally, which Kim rejected and claimed that if his crew was not allowed to evacuate to the Kraylor homeworld, he would put the Nightingale on self-destruct. (VOY: "Nightingale")