Definition for : Margin call
A Clearing house in the derivatives Market makes a margin call, i.e. a demand for additional payment, when an operator looks like it may make a loss. The operator's account is thus always in the black by at least the amount of initial deposit. If the operator does not meet a margin call, the Clearing house closes out the operator's Position and uses the deposit to cover the loss.
(See Chapter 49 Managing cash flows of the Vernimmen)
To know more about it, look at what we have already written on this subject