Celsius  (abbr.: C)adjective [ postpositive when used with a numeral]of or denoting a scale of temperature on which water freezes at 0° and boils at 100° under standard conditions.noun (also Celsius scale)this scale of temperature.USAGE Celsius, rather than centigrade, is the standard accepted term when giving temperatures: use : 25° Celsius rather than : 25°centigrade .