One decade (symbol dec) is a unit for measuring ratios on a logarithmic scale, with one decade corresponding to a ratio of 10 between two numbers.

Example: Scientific notation

When a real number like 0.007 is denoted by , it is said that the number is represented in scientific notation. More generally, to write a number in the form ''a'' × 10^''b'', where 1 <= ''a'' < 10 and b is an integer, is to express it in scientific notation, and a is called the significand or the mantissa, and b is its exponent. The numbers so expressible with an exponent equal to b span a single decade, from to .

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