Definition of diagnostic adjective from the Oxford Advanced Learner's Dictionary

diagnostic

adjective
ˌdaɪəɡˈnɒstɪk
 
; ˌdaɪəɡˈnɑːstɪk
 
 
[usually before noun] (technical) connected with identifying something, especially an illnessto carry out diagnostic assessments/testsspecific conditions which are diagnostic of AIDS