The amount of equity (collateral) required for an investment position, as a percentage of the current value.

When trading on margin (also called 'gearing', or 'leverage'), you need only deposit a fraction of the current value of the instrument you are investing in.
For example, if the commodity you are trading in requires a margin of 5%, you are able to gear (or leverage) your investment 20 times. In other words, a deposit of USD 10,000 can hold a position of USD 200,000.