noun
A person or entity that competes with another in a contest, market, or pursuit.
彼はマラソンで強力な競争者だ。
He is a strong competitor in the marathon.
新製品は市場で多くの競争者と戦っている。
The new product is competing with many rivals in the market.
相手 is a broader term for 'partner' or 'opponent' in any interaction, not necessarily competitive; 競争者 specifically implies a competitive relationship.
Compound of 競争 (competition) + 者 (person). The meaning is transparent from its components.