Any fully-automatic firearm.
The term originally applied only to larger firearms that are vehicle-mounted or fired from a tripod. However, public discourse in the 1920s and 1930s, and the subsequent National Firearms Act of 1934, has led to the term being used by the general public to include any fully-automatic firearm. Still, the term “machine gun”, when applied to a fully-automatic firearm that is hand-held, connotes a certain amount of drama and hype.