A quantity so connected with another quantity, that if
   any alteration be made in the latter there will be a consequent
   alteration in the former. Each quantity is said to be a function of the
   other. Thus, the circumference of a circle is a function of the
   diameter. If x be a symbol to which different numerical values can be
   assigned, such expressions as x2, 3x, Log. x, and Sin. x, are all
   functions of x.