What is "widal's test"

Widal's test \Wi*dal's" test\, or Widal test \Wi*dal" test\ or Widal reaction \Wi*dal" reaction\ . [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.