"Word of mouse," meaning, I think, marketing through social networking. It came from this New York Times article on social media and business travelers. It's a pretty interesting read all the way around. From the article:
Both Expedia and American Express Business Travel are announcing their new offerings this month. The Internet travel site Orbitz had one of the first networking sites, but it, too, is trying to increase participation by sending out e-mail messages to members to promote its Traveler Update.
If they do figure out a way to pull it all together, this could be terrific. We'll see how it goes.