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 a you 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.