Mark-to-market

In finance and accounting, mark to market is the act of assigning a value to a position held in a financial instrument based on the current market price for that instrument or similar instruments. For example, the final value of a futures contract that expires in 9 months will not be known until it expires. If it is marked to market, for accounting purposes it is assigned the value that it would fetch in the open market currently.
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The process whereby the book value or collateral value of a security is adjusted to reflect current market value.

The IRS's practice of calculating gains and losses on open futures positions as of the end of the tax year. In other words, taxpayers' open futures positions are marked to the market price as of the end of the tax year and taxes are assessed as if the gains or losses had been realized.