In cooking, tempering is the process of combining two ingredients of radically different temperatures. The two ingredients are slowly combined so they both gradually rise to the same temperature. Certain recipes require tempering, otherwise the shock of combining the two all at once could run the risk of the mixture curdling, seizing, lumping, or splitting. Tempering is commonly used when making sauces (adding liquid to roux), or when making ice cream and custards.
Tempering can also mean to bring an ingredient close to room temperature to shorten the required cooking time.
When working with chocolate, tempering is a completely different process. It involves stabilizing chocolate by melting it, cooling and then reheating it to specific temperatures, which makes it possible to create chocolates which are glossy and hard.
