In mathematics, proportionality indicates that two quantities or variables are related in a linear manner. If one quantity doubles in size, so does the other; if one of the variables diminishes to 1/10 of its former value, so does the other.
The symbol for proportionality resembles a stretched-out, lowercase Greek letter alpha ( ). When this symbol appears between two quantities or variables, it is read "is proportional to" or "varies in direct proportion with." Thus, the expression x y is read " x is proportional to y " or " x varies in direct proportion with y ." In this situation, as long as x and y do not attain values of zero, the quotient x / y is always equal to the same value k , which is called the proportionality constant.
Also see Mathematical Symbols .