n.  1.  One who, or that which, gives a name; origin or source of a name.  
2.  (Arith.) That number placed below the line in common fractions which shows into how many parts the integer or unit is divided.  
3.  (Alg.) That part of any expression under a fractional form which is situated below the horizontal line signifying division.

Noun  1.  denominator  the divisor of a fraction 