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When a number is changing over time, and we want to know how fast it is growing, we determine the growth rate by calculating the percentage change (% change) in the number.

% change = ^{(new value - old value)} / _{old value}

and convert into a percent (multiply by 100).

Mathematically, a short cut for this is to calculate
% change = ( ^{new value} / _{old value} ) -1

converted to percent_.
For example, if the price index for 2001 is 182 and the price index for 2002 is 186, the percentage change in the price index between the two years is
% change = ^{(186 - 182)} / ₁₈₂
or (using the short cut)
% change = ( ¹⁸⁶ / ₁₈₂ ) -1,
which is 0.021978, or 2.20% (rounding off).

The percentage change in the price index is called the "inflation rate".
The percentage change in real GDP is called the "growth rate for the economy," or "growth in (real) output," or "growth in real GDP."

Notice that you have to use this to compare two years' values. Usually the years are consecutive, which makes the growth rate an annual growth rate.
Since the numbers we use in macroeconomics, such as the price index, are computed at the end of each year, we call the growth rate the growth rate for the later year. So the value we found for inflation above (2.20%) is the inflation rate for 2002 (the later of the two years).