Learner's definition of CREDIBILITY
the quality of being believed or accepted as true, real, or honest
The new evidence lends credibility to their theory.
a lack/loss of credibility
The scandal undermined/damaged her credibility as an honest politician.
In this instance, the lawyer's job is to make the jury doubt/question the witness's credibility.
They doubted the credibility of the witness's story. [=they doubted that the story was true]