noun
or wi·dal reaction w]ēˈdal(z)-, v], -däl(z)-
Usage: usually capitalized W
Etymology: after Fernand Widal died 1929 French physician
: a specific reaction consisting in agglutination of typhoid bacilli or other salmonellas when mixed with serum from a patient having typhoid fever or other salmonella infection and constituting a test for the disease