Widal reaction | { or or } [ After Fernand Widal (b. 1862), French physician. ] (Med.) A test for typhoid fever based on the fact that blood serum of one affected, in a bouillon culture of typhoid bacilli, causes the bacilli to agglutinate and lose their motility. [ Webster 1913 Suppl. ] Variants: Widal test, Widal's test |