believability

Plausibility, as it relates to advertising copy. In terms of effectiveness, believability is actually more important than truth. The commercial or advertisement must be written within the range of possibility. In order to effect believability, the copywriter should convey the feeling of being a trusted friend. Of course, as a legal and ethical matter, the copywriter must tell the truth about the product, without overstatement or exaggeration.

noun

Appearance of truth or authenticity: color, credibility, credibleness, creditability, creditableness, plausibility, plausibleness, verisimilitude. See likely/unlikely.

The noun has one meaning:

Meaning #1: the quality of being believable or trustworthy
  Synonyms: credibility, credibleness