margin account

A brokerage account in which the broker lends the customer cash to purchase securities. The loan in the account is collateralized by the securities and cash. If the value of the stock drops sufficiently, the account holder will be required to deposit more cash or sell a portion of the stock.

Investopedia Says:
In a margin account, you are investing with your broker's money. By using leverage in such a way, you magnify both gains and losses.

Related Links:
Find out what margin is, how margin calls work, the advantages of leverage and why using margin can be risky. Margin Trading
These contracts allow for easier shorting, and provide more leverage and flexibility than stocks. Surveying Single Stock Futures

Brokerage account allowing customers to buy securities with money borrowed from the broker. Margin accounts are governed by Regulation T by the National Association of Securities Dealers (NASD), by the New York Stock Exchange, and by individual brokerage house rules. Margin requirements can be met with cash or with eligible securities. In the case of securities sold short, an equal amount of the same securities is normally borrowed without interest from another broker to cover the sale, while the proceeds are kept in escrow as collateral for the lending broker. See also Minimum Maintenance.

The noun has one meaning:

Meaning #1: an account with a securities brokerage in which the broker extends credit
  Antonym: cash account (meaning #1)