There is no single definition for drought. When a drought begins and ends is difficult to determine. Rainfall data alone won't tell you if you are in a drought, how severe your drought may be, or how long you have been in drought.
We can, however, identify various indicators of drought, such as rainfall, snowpack, streamflow, and more, and track these indicators to monitor drought.
Researchers have developed a number of tools to help define the onset, severity, and end of droughts. Drought indices take thousands of bits of data on rainfall, snowpack, streamflow, etc., analyze the data over various time frames, and turn the data into a comprehensible big picture.
A drought index value is typically a single number, which is interpreted on a scale of abnormally wet, average, and abnormally dry.