(Additional margins) A margin is collateral in the form of cash or securities deposited in an account that the holder of a position in securities has to deposit to cover the credit risk of brokers.
A broker's demand on an investor using margin to deposit additional money or securities so that the margin account is brought up to the minimum maintenance margin. Margin calls occur when your account value depresses to a value calculated by the broker's particular formula.

This is sometimes called a "fed call" or "maintenance call."
You would receive a margin call from a broker if one or more of the securities you had bought (with borrowed money) decreased in value past a certain point. You would be forced either to deposit more money in the account or to sell off some of your assets.