Precipitation reaction is a reaction in which to a substance called a precipitate separate in solid form from a solution, as by means of a reagent called precipitant.
For example:
The reaction between AgNO₃ and NaCl:
AgNO₃ + NaCl → AgCl↓ + NaNO₃
It will produce insoluble substance (AgCl) which is considered as a precipitate.