1. In common meteorological usage, a line of equal or constant value of a given quantity, with respect to either space or time; same as isogram. 2. (Also called isarithm. ) A line drawn through points on a graph at which a given quantity has the same numerical value (or occurs with the same frequency) as a function of the two coordinate variables. Compare isotimic line. 3. A straight line along which lie corresponding values of a dependent and independent variable.