A geologic (sub)Period lasting from roughly 340 million years before the present to 320 million years before the present. In North America where the interval is primarily marine limestones, it is treated as a full fledged Geologic Period between the Devonian and the Pennsylvanian. In Europe, the Mississippian and Pennsylvanian are one more or less continuous sequence of lowland continental deposits and are lumped together as the Carboniferous Period. Refer to the Carboniferous Period for more information.
There are discrepancies throughout between the articles on geologic periods and the timescale chart. That's because no two sources agree on the exact dates for geologic periods. I'll go through sometime in the next month or so and make them consistent [which is not the same as correct].