The noun rival denotes a person or thing competing with another for the same objective or for superiority in the same field of activity.

This word, first recorded in 1577, is from Middle French rival (Modern French rival) and its etymon, classical Latin rīvālis, from rīvus, a stream, and the suffix -ālis.

As an adjective, rīvālis meant of, or belonging to, a stream. As a plural noun, rīvāles designated persons who have to use the same stream, neighbours, as in the following from the digest of Justinian, a compilation of Roman law enacted under the Byzantine emperor Justinian I (circa 482-565):

Si inter rivales, id est qui per eundem rivum aquam ducunt, sit contentio de aquae usu, utroque suum usum esse contendente, duplex interdictum utrique competit.
     translation edited by Alan Watson (University of Pennsylvania Press – 1998):
If there is contention about the use of water between rivales, that is, those who draw off water through the same rivus (watercourse), with each contending that the use is his, a double interdict lies for either.

The Latin noun rīvālis came to denote one who has the same mistress as another, a competitor in love, a rival; the following dialogue between Epignomus and his slave is from Stichus, by the Roman comic playwright Titus Maccius Plautus (circa 250-184 BC):

     edited by Paul Nixon (William Heinemann – London, 1952):
– Epignomus: Vbi cenas hodie?
– Stichus:                                        Sic hanc rationem institi:
amicam ego habeo Stephanium hinc ex proxumo,
tui fratris ancillam: eo condixi in symbolam
ad cenam, ad eius conservom Sangarinum Syrum.
eademst amica ambobus, rivales sumus.
     translation by Henry Thomas Riley (G. Bell and Sons – London, 1912):
– Epignomus: Where do you dine to-day?
– Stichus: This plan have I thus resolved upon. I have a mistress here in the neighbourhood, Stephanium, the servantmaid of your brother. I’m going to invite her; I’ll take her to a pic-nic entertainment* at her fellow-servant’s, Sagarinus We both have the same mistress; we are rivals.

(* Here, pic-nic entertainment translates Latin symbola, denoting an entertainment to which each of the guests contributed in money or kind, which was the original meaning of picnic.)

The Latin proverb se amare sine rivali, literally to be fond of one’s self without a rival, meant to be alone in esteeming one’s self; in a letter of November 54 to his brother Quintus, the Roman orator, author and statesman Marcus Tullius Cicero (106-43 BC) made the following remark about C. Lucilius Hirrus:

     from Loeb Classical Library – 1972:
o di, quam ineptus, quam se ipse amans sine rivali!
     translation:
Gods, what an ass he is! How he loves himself—in which regard he has no competitor!