A submachinegun is also known as a machine pistol. The reason for this has to do with its size, orgins, and ammunition.
Submachineguns / machine pistols evolved out of automatic pistols. Both use pistol ammunition as opposed to rifle ammunition like machineguns that evolved from rifles.

Machineguns are somewhat hard to classify but generally one can use the following:

Light machinegun: Air cooled. It is portable by one man and uses basic "iron" sights. Its ammunition supply may be belt or box fed.

Medium machinegun: These are mounted on a rigid base of some sort and are crew served. The base gives the gun more stability increasing its accuracy and range. Many have more complicated sighting arrangments like telescopes or dial sights (for indirect fire) and they can be laid to fire along predetermined lines like an artillery piece. They also generally have the same caliber of ammunition as light machineguns do. Sometimes they are referred to as heavy machineguns.
They also are often water cooled instead of air cooled. This allows them to fire sustained bursts. They are almost always belt fed.

Heavy machinegun: These are usually larger caliber weapons like the M2 Browning .50 machinegun. The only real difference between a "medium" and a "heavy" machinegun is the caliber of the ammunition. Otherwise, both are identical; hence the reason mediums are sometimes conflated with heavy machineguns.