1. As a verb: to cause a dissolved substance to form a solid which comes out of solution and can be removed by settling or filtering. For example, the reduction of dissolved iron by oxidation, precipitation and filtration. 2. As a noun: the solid formed when a dissolved substance comes out of solution in such a way that it can be settled or filtered out. 3. As a verb: to cause moisture to condense and be deposited as rain, sleet, snow, etc.