Precipitation is a term used by meteorologists to describe all the type of water than can fall from the sky as weather. This includes snow, rain, sleet, and hail. Precipitation is a major part of the water cycle, and is responsible for depositing most of the fresh water on the planet. Precipitation is generated in clouds, which reach a point of saturation; at this point larger and larger droplets (or pieces of ice) form, which then fall to the earth under gravity. It is possible to 'seed' clouds to induce precipitation by releasing a fine dust or appropriate chemical into a cloud (commonly silver nitrate), encouraging the droplets to form, and increasing the probability of precipitation.
This term is also used in chemistry to describe the process whereby a solid condenses from a solution. This occurs because the solution is saturated with the solid, whereupon the solid forms, and usually sinks to the bottom of the solution.