A battle cry was a saying used to motivate oneself and/or intimidate the enemy before a conflict.
Kahless the Unforgettable was said to have the battle cry "Today is a good day to die." (VOY: "Dragon's Teeth")
Another old Klingon battle cry spawned the favorite saying of Lyndsay Ballard, "Own the day." (VOY: "Ashes to Ashes")
During the Federation-Klingon War of 2256–57, the Klingon battle cry was "tlhIngan maH taHjaj", which translated to "Remain Klingon," or more properly "We are Klingon, may we endure." The phrase originated from T'Kuvma, who had contrived the war to unify the Klingon Empire against a common foe. In 2259, Lieutenant Erica Ortegas recounted hearing the cry over the comms before the Klingons attacked. (DIS: "The Vulcan Hello", "Battle at the Binary Stars"; SNW: "Under the Cloak of War")
In 2372, Kor told a story of Koloth chanting a battle cry as he attacked the flanks of T'nag's army with Kang and Kor. (DS9: "The Sword of Kahless")