A pacer can refer to several different things depending on the context, but generally it means:
A device used in medicine to regulate the rhythm of the heart: A pacer, short for pacemaker, is a small device implanted in the chest that sends electrical impulses to the heart muscle to regulate its rhythm and ensure proper beating.
An athlete who sets the pace in a race: A pacer can also refer to a runner who sets the pace for a group of athletes in a long-distance race to help them maintain a consistent speed and achieve their target time.
A mechanical device used in construction: A pacer is a mechanical device used in construction to compact soil, gravel, or asphalt to create a solid and stable foundation for buildings, roads, or other structures.
A horse used for pacing: In horse racing, a pacer is a type of horse that moves both legs on the same side of its body forward at the same time, rather than the more common trotting gait where the horse moves its diagonal legs together.
A tool used to measure distance: In sports such as track and field, a pacer can refer to a tool used to measure the distance covered during training or competition.