My definition of a fast prime is more intuitive than a dictionary definition.
I'd say any normal length lens below f2.8 is a fast prime, but the 300mm f2.8, 500mm f4, and 600mm f4 are all also fast primes.
Below are photos of my three fast normals, a zeiss 55mm f1.4, a rokinon 24mm f1.4 and a canon 35mm f2.0 IS all on the same subject but with size controlled by foot action. BTW illumination is provided by an LED panel to the left.
http://www.canonrumors.com/forum/index.php?action=gallery;sa=view;id=114
http://www.canonrumors.com/forum/gallery/0/57161-070317021520-1121588.jpeg
http://www.canonrumors.com/forum/index.php?action=gallery;sa=view;id=112
I'm not in love with the new way of adding photos BTW