tes·ti·fy 
/ˈtɛstəˌfaɪ/ verb
tes·ti·fies; tes·ti·fied; tes·ti·fy·ing /ˈtɛstəˌfaɪ/ verb 1 a : to talk and answer questions about something especially in a court of law while formally promising that what you are saying is true [no obj] ▪ She refused to testify about who had given her the information. ▪ Three witnesses were called/summoned to testify at (the) trial. ▪ She testified before Congress today. ▪ He agreed to testify against his drug dealer. ▪ They will testify for the defense/prosecution. [=they will testify because the defense/prosecution has asked them to] [+ obj] ▪ He testified that he'd seen two people leave the building on the night of the murder.
b : to talk about or say (something) in an honest and confident way [+ obj] ▪ I can (personally) testify that the food at that diner is excellent. [no obj] — + to ▪ Many of her former employees came forward to testify to her generosity. [=to say that she is a generous person] 2 : to show that something is true or real : to give proof of something [no obj] — + to ▪ These statistics testify to the fact that the program is working. [=show that the program is working] ▪ The fact that doctors were able to catch the disease before it had spread testifies to the importance of medical screenings. [+ obj] ▪ These statistics testify that the program is working. 3 [no obj] US : to talk to a group of people about your belief in God especially as part of a church service
▪ Several people testified during the revival meeting.
b : to talk about or say (something) in an honest and confident way [+ obj] ▪ I can (personally) testify that the food at that diner is excellent. [no obj] — + to ▪ Many of her former employees came forward to testify to her generosity. [=to say that she is a generous person]
▪ Several people testified during the revival meeting.
