Cavalry was a type of military unit where the soldiers usually travelled and fought on horses.
Cavalry units had the speed and mobility to attack enemy troops behind the lines and avoid capture in a tactic known as cavalry raid. (DS9: "Once More Unto the Breach")
The Colt .45 was a cavalry pistol invented by Colt Firearms in 1873. (TNG: "Time's Arrow")
The US 7th Cavalry Regiment was a cavalry unit of the United States Army. (TNG: "The Defector")
In 2267, while on Capella IV, Captain Kirk commented that the "cavalry doesn't come over the hill in the nick of time anymore", when he couldn't reach the USS Enterprise to call for help. (TOS: "Friday's Child")
When aboard the Krayton in 2366, William T. Riker told Lwaxana Troi to keep DaiMon Tog occupied while he called the cavalry (meaning the USS Enterprise-D). (TNG: "Ménage à Troi")
Similarly, in 2373 Captain Janeway ordered Harry Kim to "send in the cavalry", which meant creating false echoes of ten Talaxian ships to distract attacking Kazon ships. (VOY: "Basics, Part I")