Simmering is a food preparation technique by which foods are cooked in hot liquids kept just below the boiling point of water (lower than 100 °C or 212 °F) and above poaching temperature (higher than 71–80 °C or 160–176 °F. To create a steady simmer, a liquid is brought to a boil, then its heat source is reduced to a lower, constant temperature.

Source: Wikipedia