Source : Webster's Revised Unabridged Dictionary (1913)

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