Money borrowed from a broker to trade, or the collateral you must post to hold a leveraged position.
Margin and leverage. A $5,000 deposit can control a $100,000 position, which is 20:1 leverage. Because the loss is measured on the full $100,000, a 2.5% move against you halves the deposit and brings a margin call, and a 5% move uses all of it.
In stocks, margin is a loan: you put up 50% under regulation-t and borrow the rest, paying interest. In futures and forex, margin is a performance bond, not a loan: the initial-margin you post to open a contract.
Example: with $10,000 and 2:1 stock margin you can buy $20,000 of stock. A 10% drop costs $2,000, which is 20% of your money, plus interest on the $10,000 borrowed.