rate of fire is an important characteristic of firearms. It is the number of times that the weapon can shoot in a unit of time. It is often expressed in rounds per minute (abbreviated as rpm), and qualified by stating firing conditions.
For example the 2S19 MSTA-S 152-mm Self-Propelled Howitzer, a Russian field gun has a rate of fire of 8 rounds per minute (burst), 6 (normal) and 2 rounds per minute sustained.
In the specific context of automatic weapons, normally a distinction is made between the cyclic rate of fire, which is the number of rounds that the weapon mechanism is able to fire in a minute (without taking into account the need for reloading and other various practical problems), and the practical rate of fire, which is what the weapon, in the hands of an expert operator, is capable of doing in real life conditions.
For example, the M16A1 automatic rifle has a cyclic rate of fire of 800 rpm but a practical rate of fire of 90 rpm in burst mode and 45 rpm in semiautomatic mode; the limited magazine capacity and the risk of overheating limit the rate.
most information from http://www.fas.org/