Cavalry units are organized into squadrons, further divided into troops, hence a trooper is a member of a troop. "Trooper" can also be used colloquially to mean any cavalry soldier (although not usually an officer).
In the United States Cavalry and airborne forces, "trooper" is a colloquialism that has traditionally been used not as a rank, but rather as a general term for any enlisted soldier.[3]
Cavalry Troopers are generally considered to be socially a cut above other soldiers. This distinction stems from the days when cavalry needed to supply their own horses and equipment, and so would need to be reasonably wealthy and a gentleman of sorts. In addition cavalry regiments were seen to be relatively fashionable and dashing, often having colourful or even garish uniforms.
In the United States many states have a state branch of law enforcement that serves as a highway patrol. Many state highway patrols refer to their officers as troopers.