a term referring to the public discussion or buzz that a film can acquire, fueled by sneak previews and advance advertising; word of mouth is an important marketing element in a film's success or [..]

Word of Mouth or word-of-mouth (WOM) marketing or communication is the passing of information about a brand’s product or service from one read more:

the unofficial channel of correspondence wherein facts, rumors, opinions, and gossip are conveyed by speaking from one individual to another.

Word of mouth is a formalized marketing effort designed to stimulate the passing of information by verbal means, especially recommendations, but also general information, in an informal, person-to-per [..]

referral.

The passing of information from person to person by oral communication. In the modern sense, Word of Mouth includes mobile and social technologies. (Wikipedia – Word of Mouth)

(idiomatic) Verbal means of passing on information. ''The book didn't need to be advertised; it became popular solely by word of mouth.''