Decimal expansion refers to the representation of a number in the base-10 numeral system, which uses digits from 0 to 9. This method allows for both whole numbers and fractions to be expressed in a format that is easy to read and understand. For example, the fraction 1/2 can be represented as 0.5 in decimal form. In decimal expansion, numbers can be finite or infinite. Finite decimals have a limited number of digits after the decimal point, like 0.75, while infinite decimals can either be repeating, such as 0.333..., or non-repeating, like the decimal expansion of π (pi), which is approximately 3.14159.