The term "mark-to-market" refers to the process of determining the value of all open positions in a stock or commodity. This is typically done at the end of each trading day, using the closing price as the benchmark. This practice allows for accurate and up-to-date assessment of the value of these positions, providing important insights for investors and traders alike. In essence, mark-to-market is a crucial tool in the world of finance, helping to inform decisions and manage risk.