Two variable quantities are said to be proportional to one another if their quotient has a fixed value m. m is called the proportionality factor. For instance, the ratio of current to voltage is constant for a direct current (Ohm's law).

 

Two numbers are said to be directly proportional if they stand in direct ratio to one another, e.g. time worked and money earned (the longer the time worked the more money earned).

 

Two numbers are said to be inversely proportional if they stand in reciprocal (indirect) ration to one another, e.g. speed, and time taken to travel a certain distance at that speed (the greater the speed, the shorter the time taken).