A decimal is a fraction written in a special form. Instead of writing 1/2, for example, you can express the fraction as the decimal 0.5, where the zero is in the ones place and the five is in the tenths place.
Decimal comes from the Latin word decimus, meaning tenth, from the root word decem, or 10. The decimal system, therefore, has 10 as its base and is sometimes called a base-10 system. Decimal can also specifically refer to a number in the decimal system. As an adjective, decimal means something related to this numbering system. The decimal point, for example, refers to the period that separates the ones place from the tenths place in decimal numbers.