IPA: /ˈɑːrˌraɪvəl/
KK: /ˈɑrˌraɪvəl/
A person or group that competes with another in a significant way, often seen as the main opponent.
The two teams have been archrivals for decades, always competing for the championship title.
Archrival → It is formed from "arch-" (from Greek "archos", meaning chief or principal) and "rival" (from Latin "rivalis", meaning one who competes for the same thing). The word "archrival" refers to a principal or chief competitor, often the most significant opponent in a rivalry.
Think of 'arch-' meaning chief and 'rival' meaning competitor — an archrival is your chief competitor.