An antibody to a soluble antigen that specifically aggregates the macromolecular antigen in vivo or in vitro to give a visible precipitate.

These type of reactions occur only with multivalent antigens and are dependent on electrolyte concentration, pH,temperature and the relative concentrations of antigen and antibody, with the amount of precipitate formed increasing to a maximum and the decreasing as the relative antigen concentration is increased.

In layman's terms:
When an antibody binds to an antigen, this particular type of reaction produces huge molecules that become insoluble and therefore become visible to the human eye. This makes the overall liquid appear cloudy.