A fraction from the Latin fractus, is a number that can represent part of a whole.
The earliest fractions were reciprocals of integers, symbols representing one half, one third, one quarter, and so on. A much later development were the common or “vulgar” fractions which are still used today, and which consist of a numerator and a denominator. An example is 3/4, in which the numerator, 3, tells us that the fraction represents 3 parts, and the denominator, 4, tells us that 4 parts make up a whole.
A still later development was the decimal fraction, now usually called simply a “decimal”, in which the denominator is a power of ten, determined by the number of digits to the right of a the decimal point. Thus in 0.75 the numerator is 75 and the denominator is 10 to the second power because there are two digits to the right of the decimal and thus the denominator is 100.