Water freezes at 0 degrees Celsius under standard atmospheric pressure.
The Celsius scale sets the freezing point of water at 0 °C and its boiling point at 100 °C, with both reference points defined at standard atmospheric pressure. The scale divides the interval between those points into 100 equal degrees, which is why it was formerly called the centigrade scale.
The freezing point is not completely independent of conditions. Pressure, dissolved substances, and impurities can change the temperature at which water begins to freeze. Salt lowers water’s freezing point, which is why salt is spread on icy roads and why seawater freezes below 0 °C. At higher elevations, atmospheric pressure is lower, but the effect on water’s freezing point is much smaller than the effect on its boiling point.
Another useful distinction is between freezing and melting: for pure water at a given pressure, the solid-to-liquid and liquid-to-solid transitions occur at the same equilibrium temperature. The Fahrenheit equivalent of 0 °C is 32 °F.