An Etymological Dictionary of Astronomy and Astrophysics

1. A person who is competing for the same object or goal as another, or who tries to equal or outdo another; competitor.
   

2. A person or thing that is in a position to dispute another’s preeminence or superiority (Dictionary.com).

Etymology (EN): From L. rivalis “a rival, adversary; neighbor,” originally, “one who uses a stream in common with another,” from riv(us) “stream, brook,”

• -alis, → -al.

Etymology (PE): Hamâvard “a rival; an adversary in a combat,” from ham- “together,” → com-, + âvard “battle, struggle,”
variants nabard, nibard, nâvard “fight, struggle, war,” ultimately from Proto-Ir. *part- “to fight, to struggle.”