It stands for mark-to-market margin; with respect to buying on margin, it is the minimum amount of equity which an investor must maintain in a margin account. Generally, investors who buy securities on margin are required to deposit a margin of 25%-40% of the total market value of those securities.

In futures markets, mark-to-market margin refers to the minimum amount which is set by futures exchanges to be maintained daily by the parties to a futures contract in their margin accounts, in response to fluctuations in the settlement price. As long as the margin account is maintained at or above the mark-to-market margin, a futures position can still be held. If the margin account drops below the mark-to-market margin, a margin call would be sent to the futures holder to top up his margin account with more cash or marketable securities in order to raise the margin level back to normal. The margin can also be brought back to the desired level by taking offsetting positions in open futures contracts. The mark-to-market margin level is usually set slightly below the initial margin.

It is also known as a maintenance margin.