margin call

A procedure related to the application of variation margins, implying that if the value, as regularly measured, of the underlying assets falls below a certain level, the central bank requires counterparties to supply additional assets (or cash). Similarly, if the value of the underlying assets, following their revaluation, were to exceed the amount owed by the counterparties plus the variation margin, the counterparty may ask the central bank to return the excess assets (or cash) to the counterparty.
See also:
 repurchase agreement     
 variation margin (marking to market)