A security that one has purchased or sold on a margin account. A margin account is a brokerage account in which the brokerage lends the account holder money, which the account holder then uses to buy securities. Thus, a margin security is one that an investor buys with borrowed money. The fact that an investor is able to do this opens up investment opportunities that he/she might not otherwise be able to afford. More importantly, however, a margin security increases the possibility of a higher return and the risk of more losses. Margin securities are governed by Regulation T. See also: Margin call, Maintenance.