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Definition: A number expressed using the decimal number system. Commonly, the term is used for numbers that have fractional parts indicated by a decimal point. A number less than 1 is called a decimal fraction; for example, 0.537 is a way of writing 0+(5/10)+(3/100)+(7/1000)
Source: Nelson, David. The Penguin dictionary of mathematics 4th ed. London, New York. Penguin. 2008.