between an antibody
and an antigen
during an immunology lab procedure
where the two are allowed to diffuse
toward each other through an agar
medium. Lines of precipitation
form in the places on the gel where the two react
with each other and shows where the reaction has occurred.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.