Market Challenger

Market challenger
A business in a market that is fighting hard to increase its market share
Market concentration ...

Market Challenger a company holding a major market share and competing vigorously with the market leader for outright leadership. See Market Follower; Market Leader; Market Nicher.

Encirclement Attack - a competitive strategy used by a strong challenger to attack the market leader; the market challenger launches an attack on several fronts at once in an attempt to break the leader's grip on the market.

a company's industry ranking by size and business strength, may be classified as a market leader, market challenger, market follower, or market nicher, according to the market share it holds
competitive scope ...

See also: Firm, Leader, Consumer, Market Leader, Profit