50cc is a measurement of 50 cubic centimeters. 50cc is approximately 3 cubic inches.
50cc is the most common legal displacement classification of moped, scooter, motor-assisted cycle and other such vs being considered a motorcycle in most states.
In many states, a VIN, up to 50cc displacement along with the presence of operable pedals, automatic transmission of power and horsepower limitations are the key requirements to be qualified as a "moped".
The exact technical factory cylinder displacement varies by make, model and year of most mopeds, typically ranging from 47.xx cc through 49.xx cc but are MCO'd and VIN'd as "50cc"