Any surfboard that is more than 3' longer than the rider is defined as a Lonboard by the Hawaians.
HOWEVER - it is more the state of mind.
Longboarding is about the soul, expression and The Glide. Where shortboarders beat each other for the waves and perform aggresive moves to carve up the water, you will almost certainly find a longboarder behind them, contemplating their surroundings, taking in the view and with a huge grin on their face.
Add another longboarder and you have a friend with a common goal; glide, float, dance and shimy on the board using your board and your body together - swap stories of shortboarders scattering in your wake and the time you soul arched hanging ten for 20 seconds at Jeffries..
Longboarding is the original soul of original surf culture.
Man I was out the other day on my 10'4" longboard and the waves were 2' and clean - best waves I ever had - shortboarders couldn't catch 'em - have you ever seen a Longboader whinge that there is no surf?