Margin

Cash held back as a deposit against a trade that can lose more than you put in. Here, margin is held from your cash balance when you sell short or sell an uncovered option, and it is released when you close the position. Selling a share short holds 50% of its value, and an uncovered option holds 20% of the underlying's value for each contract. It is still part of your total value, but you cannot spend it on other trades. If short losses push your cash balance below zero, that is a margin call: you cannot open new positions until the balance is positive again.