Wiktionary
immunodiffusion
n. a technique used to detect the reaction between an antigen and an antibody by observing the behavior of a combination of such species as they diffuse through a gel
Wikipedia
Immunodiffusion
Immunodiffusion is a diagnostic test which involves diffusion through a substance such as agar.
Two commonly known forms are Ouchterlony double immunodiffusion and radial immunodiffusion.