Decimal is a numeral system with a base 10 because it has 10 characters (called digit):
The usual decimal representation of whole numbers gives every whole number a unique representation as a finite sequence of digits.
When decimal representation is used for the rational or real numbers, such numbers in general have an infinite number of representations, for example 2.31 can also be written as 2.310, 2.3100000, 2.309999999…, etc., all of which have the same meaning except for some scientific and other contexts where greater precision is implied by a larger number of figures shown.