Being included and traded on a given exchange. Most exchanges have specific requirements which companies must meet in order to be listed and continue to stay listed.
Investopedia Says:
Companies are frequently being added to a given exchange, such as the Nasdaq. And occasionally companies that have not fulfilled all necessary listing requirements become delisted for a period of time until they again meet the requirements. Generally, companies prefer to be listed on the major exchanges, such as the NYSE and Nasdaq, since they provide the most liquidity and visibility for a company's stock.
Related Links:
Find out the answers to all the questions you had about stock exchanges but were afraid to ask! Getting to Know Stock Exchanges
Listed securities are 'the cream of the crop'. Find out how a firm can lose that status and why you should be wary. The Dirt On Delisting
