Ultralight Vehicles:

An Ultralight vehicle, according to Federal Aviation Regulation 103.1
is a vehicle that:

a) Is used or intended to be used for manned operation in the air by
   a single occupant.
b) Is used or intended to be used for sport or recreation only.
c) Does not have any U.S. or foreign Airworthiness Certificate; and
d) If unpowered, weighs less than 155 pounds; or
e) If powered:
   1) Weighs less than 254 pounds empty weight, excluding floats and
      safety devices intended to be deployed in a potentially catastrophic 
      situation;
   2) Has a fuel capacity not eceeding 5 U.S. gallons
   3) Is not capable of more than 55 knots calibrated airspeed at full 
      power in level flight; and
   4) Has a power-off stall speed which does not exceed 24 knots 
      calibrated airspeed.

Notice they never call the thing an aircraft??? Always a "vehicle".