A numerator is the top part of a fraction, which indicates how many parts of a whole are being considered. For example, in the fraction 3/4, the number 3 is the numerator, showing that three out of four equal parts are being referenced. The numerator works in conjunction with the denominator, which is the bottom part of the fraction. The denominator represents the total number of equal parts in the whole. Together, the numerator and denominator help express ratios, proportions, and divisions in mathematics.