Which culinary technique involves concentrating flavors by simmering ingredients over a low heat for an extended period?

The technique that involves concentrating flavors by simmering ingredients over a low heat for an extended period is reduction. When cooking through reduction, the moisture content in a sauce or liquid is intentionally evaporated, which intensifies the flavors and creates a thicker consistency. This method is frequently used in sauces and soups, allowing the individual components' flavors to meld together harmoniously, resulting in a more robust and savory final dish.

While clarification focuses on removing impurities to create a clear liquid, deglazing is the process of adding liquid to a pan to dissolve the flavorful browned bits left after sautéing or roasting. Infusion involves soaking ingredients in a liquid to extract flavors, but it does not necessarily concentrate them through evaporation in the same way that reduction does.