Maintenance margin refers to the minimum amount of equity that an investor must maintain in their margin account to avoid a margin call from their broker. The maintenance margin is typically set by the broker and is expressed as a percentage of the total value of the securities held in the account.

For example, if an investor has $10,000 worth of securities in their margin account and the maintenance margin requirement is 25%, the investor must maintain at least $2,500 in equity in their account. If the value of the securities in the account drops below a certain level, the broker may issue a margin call, requiring the investor to deposit additional funds to bring the account back up to the required maintenance margin level.