The Collaborative International Dictionary of English v.0.48:

margin call \margin call\ n. (Stock market)
   A demand by a broker that a customer deposit enough to bring
   his margin up to the minimum requirement; -- caused by the
   decline in market prices of a security or commodity purchased
   on margin[5].

   Syn: call.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

margin call
    n 1: a demand by a broker that a customer deposit enough to
         bring his margin up to the minimum requirement [syn:
         margin call, call]