重机枪是机枪的两个分类中较大型号那类的统称（相对于轻机枪）。这个词原本是指那些在第一次世界大战中发射枪弹（.30和7.62毫米），拥有紧密的支架结构以及拥有水冷设备的重型支撑型防御武器。但是此类武器由于过于笨重，难以移动而最终被弃。那段时间，“重”在“重机枪”中的意思并不是枪管口径（10毫米以下口径的比比皆是）而是其连续发射枪弹的能力，通常定义为三脚架、水冷式、弹链供弹。这类武器中最好的例子是由美国人马克沁发明的马克沁机枪。在第一次世界大战中，三个独立的敌对国家使用的机枪都是马克沁机枪的变种（德国的MG08重机枪，英国的维克斯机枪和俄国的M1910重机枪）。

The heavy machine gun or HMG is a class of machine gun implying greater characteristics than medium machine guns.

There are two generally recognized classes of weapons identified as heavy machine guns. The first is weapons from World War I identified as "heavy" due to the weight and encumberment of the weapons themselves. The second is large-caliber (generally .50 or 12.7mm) machine guns, pioneered by John Moses Browning with the M2 machine gun, designed to provide increased range, penetration and destructive power against vehicles, buildings, aircraft and light fortifications beyond the standard rifle calibers used in medium or general-purpose machine guns, or the intermediate cartridges used in light machine guns.