Definitions:

1. Group Activities or Support: In group fitness or rehabilitation settings, a pacer might refer to a person who leads exercises or activities at a specific pace to accommodate the abilities of all participants. NOUN
   • In the fitness class, the instructor acted as a pacer, ensuring the workout was accessible to beginners and more advanced members alike.
2. In Horse Racing: A pacer, also known as a standardbred, is a breed of horse that competes in harness racing, moving at a pace (a lateral gait) as opposed to a trot. NOUN
   • The pacer seemed in perfect harmony with its driver, gliding around the track with powerful strides.
3. In Sports and Athletics: A pacer is an individual or a device used to set and maintain a specific speed or pace during a race or training session. NOUN
   • The marathon organizers provided pacers for runners aiming to finish within four hours, helping them keep their pace steady throughout the race.
4. Medical Device: In the medical field, a pacer can be shorthand for a pacemaker, which is a device implanted in the chest to regulate abnormal heart rhythms. NOUN
   • After the surgery to implant the pacer, his heart rhythm stabilized, greatly improving his quality of life.
5. Public Transportation: The term "Pacer" is used in some regions to refer to a type of train or tram that sets or maintains a consistent speed along a transit line. NOUN
   • The old Pacers were finally being phased out, replaced by more modern and efficient trains.

Synonyms:

• pacemaker
• pacesetter