"Word of mouth" could mean it is a rumour but it doesn't have to be. It is a little more general and simply means that information has been received without any documented confirmation. The documentation might exist, but the person who received the information by word of mouth hasn't seen it.

I would just add that it is information "heard", passed along from person to person, "by mouth". With regard to business, it may be the manner by which the reputation of a merchant is spread, without the use of media advertising, to gain (or lose) customers.

you might mean "word of mouth" it means something that alot of people are talking about, but has not really been proven to be true yet. It's the same meaning as a RUMOR. example: WORD OF MOUTH IS, they're making the stocks public next month. What the speaker is trying to say here is that people have been talking about the stocks being made public by next month, but nobody has been able to verify if the information is correct. another way to say this is: RUMOR HAS IT that the stocks will be made public next month.