It’s a simple word, credibility. Basically, it means a person’s believability. A person may not be credible because he has a history of lying, or because what he says doesn’t make sense when taken with other more believable facts.
In our recent trial, which was a liability dispute with no independent witnesses (we commonly call these “he-said, she-said” cases, regardless of gender), every single lawyer used the word credibility in opening statement and closing argument. Essentially, we all argued that our clients were credible, that the other side did not testify credibly, and sometimes that the witnesses were somewhat credible. It’s standard fare for a trial.
In a trial, credibility is the most important thing a witness or a party has. If the judge or jury has any reason to doubt that person’s truthfulness on any single point, even something unrelated, that doubt can cast a shadow on every point of that person’s testimony. That’s why some lawyers will fight hard to find a lie or a mis-rembering in testimony. It can be the difference between a win and a loss.