Rainfall is generally measured by a rain-gauge:
Tipping Bucket Rain-gauge
Definition of precipitation
Precipitation is defined as the liquid or solid products of the condensation of water vapour falling from clouds or deposited from air on the ground. It includes rain, hail, snow, dew, rime, hoar frost and fog precipitation which reaches the ground. It is expressed in terms of the vertical depth of water (or water equivalent in case of solid forms) to which it would cover a horizontal projection of the Earth's surface. Snowfall is also expressed by the depth of fresh snow covering an even horizontal surface.
Precipitation gauges (often known as rain-gauges) are the most common instruments used to measure precipitation.
Precipitation measurements are particularly sensitive to exposure, wind and topography. Any method of measuring precipitation should aim to obtain a sample which is representative of the true amount falling over the area which the measurement is intended to represent. A precipitation gauge should be installed on level ground. Its distance from every surrounding object should be at least twice the height of the object above the rim of the gauge.
Unit of measurement
Millimetres (mm) for liquid precipitation (rainfall).