A market where there is a threat of competition, not necessarily actual competition, but the threat could be enough to affect the behavior of firms in the market
Characteristics of a contestable market

Lower barriers to entry and exit
Large pool of potential entrants
Good information of market conditions so new firms can compete on a level playing field
Incumbent firms are subject to hit and run competition due to low barriers to entry and exit
Technology
Has increased the contestability of markets by reducing barriers to entry and exit, allowing for greater innovation and disruption, and improving information
Behavior of firms in a contestable market
1.Monopolist lowers price and increases quantity to the limit price (where average cost equals average revenue) to eliminate the threat of entry and competition
2. Monopolist is prepared to compete with lower prices and higher quantities if entry does occur