Mark to the market

When an investment is marked to the market, its value is adjusted to reflect the current market price.

With mutual funds, for example, marking to the market means that a fund's net asset value (NAV) is recalculated each day based on the closing prices of the fund's underlying investments.

With a margin account to buy futures contracts, the value of the contracts in the account is recalculated at least once a day to determine whether it meets the firm's margin requirements.

If that value falls below the minimum specified, you get a margin call and must add assets to your account to return it to the required level.