A measure that the Bureau of Labor Statistics (BLS) defines as the average change over time in the prices paid by consumers for a market basket of goods and services. This number is used to define the inflation percentage over time.
The CPI uses a base value of 100 for a given number of goods and services and uses the years 1982-1984 as its base years. The BLS then calculates what the same number of goods and services would cost each year. For example the Feb 2019 index (for all goods) was 252.776 and the Feb 2020 index was 258.678. By dividing the two numbers we see that the inflation in this 12 month period is a 2.3% increase.