A clinical lab test used to detect the presence of a certain hemagglutinating virus or other hemagglutinin antigen based on whether the red blood cells in the sample lose the ability to clump together when the antibody to the virus or other antigen is added to it. If the virus or antigen is present, the antibody kills it and thereby stops it from being able to stick the red blood cells to each other.
This is also known as a hemagglutination inhibition assay, HI test, or HAI test.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.