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1.
The inability or refusal to listen to one's own instinct or 'gut feeling' but to instead follow the majority for fear of being wrong, ostracized or ridiculed.
If a herd of animals were haded for a cliff, for the sake of a herd mentality, I doubt one would veer off or stop to save its own life.
by quietstorm October 16, 2007
 
2.
A very common disease that is often displayed in Politically-Correct People.

One typical example of this is when a chorus sings a patriotic song such as "The Star-Spangled Banner", the audience thinks they should stand up and sing along just in case anyone wants to question their patriotism. Quite hard to cure, highly amusing.
Random movie theater audience member: Wow, that was such a great movie that I think I'll get up and clap!

*the whole audience notices and does the same*

Another more sensible audience member: ...They do know that they're applauding at an inanimate object, right?

Me: Poor souls, they've been inflicted with Herd Mentality.
by EniBelle April 12, 2010
 
3.
A sales, marketing, and business term describing why customers buy products or services because their friends, colleagues, neighbors, and family bought those same products and services previously. Also applies to a buyer's selection of a supplier or sales person to provide products and services to them.
Five years ago, our home had the only swimming pool on Apple Street. Now, thanks to herd mentality, over half the homes on Apple Street have a swimming pool.