The dollar amount required to be kept at the OCC throughout the life of an option contract; percentage of the dollar amount of securities that must always be kept as margin.
The minimum margin which an investor must keep on deposit in a margin account at all times. 
The lowest balance of funds that a broker will allow a counterparty to maintain when trading on... <a target=_blank href='http://www.finance-glossary.com/terms/maintenance-margin.htm?id=12727&ginPtrCode=00000&PopupMode=false' title='Read full definition of maintenance margin'>more</a>