A precipitate? In layman's terms, it is a substance that is insoluble (can't be dissolved) in water.
It is often formed from a substance in solution by allowing it to react with something that is added to the solution. For example, calcium chloride, CaCl2, is quite soluble in acidic water solutions, but calcium hydroxide, Ca(OH)2 is not. If you add a solution of a strong base, say sodium hydroxide (NaOH) to a calcium chloride solution, you will form calcium hydroxide that will "precipitate out" (form an insoluble precipitate that will fall to the bottom of the container, or can be filtered off).
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