Definition: A hardtail mountain bike is simply a mountain bike without rear suspension.
Originally, no mountain bikes had any rear suspension. So in reality, they were all hardtails. It's a little funny that now that most serious mountain bikers ride on bikes with rear suspension, the term hardtail was created to describe these original bikes without rear suspension, but that is how it goes.
Also Known As: stiffys, rigid, uncomfortable
Examples: I don't like hardtails so much because thedy are less comfortable and can be more difficult to ride in some conditions.