Wikipedia
The Trigarium was an equestrian training ground in the northwest corner of the Campus Martius ("Field of Mars") in ancient Rome. Its name was taken from the triga, a three-horse chariot.
The Trigarium was an open space located south of the bend of the Tiber River, near the present-day Via Giulia. It may be part of a larger field set aside as a public space for horse pasturage and military drill for youths, which was the original purpose of the Campus Martius. The earliest reference to the Trigarium dates to the time of Claudius, and the latest to the second half of the 4th century.
To preserve its flexibility of purpose, the Trigarium had no permanent structures; it was used for chariot training and all forms of equestrian exercise. The faction headquarters of the professional charioteers were established nearby, with the Trigarium just northwest of the stables and clubhouse of the Green and Blue teams. An adjacent area where people played ball and hoop games and wrestled was the site of temporary wooden stadia built by Julius Caesar and Augustus and finally the permanent Stadium of Domitian.
Trigarium became a generic word for an equestrian training ground, as evidenced by inscriptions. For instance, a charioteer in Roman Africa who died during a race was buried in the nearby trigarium. Pliny uses the word to mean equestrian exercise generally: he describes a fortified water or sports drink, prepared with powdered goat dung and vinegar, that was drunk by Nero "when he wanted to strengthen himself for the trigarium." Pliny asserts that Italian horses were superior for the exercises of the trigarium.