Imperfect competition

Imperfect competition is a competition that does not fit the definition of perfect competition either because it involves a smaller number of firms or only one firm, or products that aren't identical.

Definition

According to Principles of Economics by Timothy Taylor (3rd edition),

Imperfect competition. Competition that does not fit the definition of perfect competition either because it involves a smaller number of firms or only one firm, or products that aren't identical.