For SLR cameras (film 24x36mm, 42mm diagonal) is an f = 42mm a normal lens. Previously f = 50mm lenses are called normal lens (well, f = 50mm was close to f = 42mm)
Half-inch lenses of f = 8mm are normal lenses.
One-inch lenses of f = 16mm are normal lenses.
Normal lenses help to define “wide angle lens” and “telephoto lenses”:
Lenses with a focal length less than that of a normal lens is called wide angle lens.
Lenses with a focal length greater than that of a normal lens are called telephoto lens.