In modern record-keeping, rainfall is measured by the amount in a given time period, usually hourly or daily. A few drops of rain is called a “trace” of rain. Rainfall is “measurable” if it adds up to 0.01 inch (0.25 millimeters) or more.
If you want to know how much it rains where you live, the best way is to install a rain gauge and start recording your own daily rainfall measurements. A great resource is the CoCoRahs Network, a community of volunteers working to measure and map rain, hail, and snow.