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

diagnostic

noun
ˌdaɪəɡˈnɒstɪk
 
; ˌdaɪəɡˈnɑːstɪk
 
 
(computing)1 (also diagnostic program) [countable] a program used for identifying a computer fault2 [countable] a message on a computer screen giving information about a fault3 diagnostics [uncountable] the practice or methods of diagnosis