contestability

For a contestable market to exist there must be low barriers to entry and exit so that there is always the potential for new suppliers to come into a market to provide fresh competition to existing suppliers. For a perfectly contestable market, entry into and exit out of the market must be costless
The reality is that no market is perfectly contestable (there are always some “barriers to contestability”). That said it is also true that virtually every market is contestable to some degree even when it appears that the monopoly position of a dominant seller is definite.

Contestable markets and perfect competition - the differences
Contestable markets are different from perfect competitive markets. E.g. it is feasible in a contestable market for one firm to dominate the industry, haveprice-setting power and also for firms in a market to produce adifferentiated product both of which run counter to the assumptions behind the traditional model of perfect competition. 
There are three main conditions for pure market contestability:

1. Perfect information and the ability and/or the right of all suppliers to make use of the best available production technology in the market
2. The freedom to market / advertise and enter a market with a competing product
3. The absence of sunk costs – this reduces the risks of coming into a market