What is a solid formed from liquid reactants during a chemical reaction?
It is called a precipitate.
Compounds in solution may react to form other compounds whose solubility in the liquid medium is less than that of the original compounds. When this occurs the new compound drops out of the solution as a solid.
An example would be the reaction of silver nitrate and sodium chloride in water. The resulting silver chloride will precipitate out of the solution as a solid.