The Margin call reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Margin call

A margin call is the demand, by a broker, for additional funds, additional money or securities, because of an adverse price movement in one or more of your held securities to bring a margin account up to the minimum maintenance margin. Because the securities held in a margin account are acquired partially with borrowed money, the broker has the right to demand that you maintain this minimum balance in your account. Margin calls occur on long positions when the value of held securities falls, and on short positions when the value of held securities rises.

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