(general) The act of clumping together or adhesion of particles.
Biological agglutination occurs in (but not limited to) the following instances: (1) clumping of cells in response to an antibody, (2) the adhesion of small particles that are suspended in a solution, which are then (usually) precipitated (3) in allergic reactions, when cells clump together to prevent antigens from entering.
Word origin: from the Latin agglutinare, meaning "to glue to"
Related forms: agglutinin (noun), agglutinate (verb).
Related terms: cross agglutination, direct agglutination, false agglutination, group agglutination, indirect agglutination, mixed agglutination, passive agglutination, spontaneous agglutination.