A meander cutoff occurs when a meander

Meander

A meander in general is a bend in a sinuous watercourse. A meander is formed when the moving water in a stream erodes the outer banks and widens its valley. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the...

 bend in a river is breached by a chute channel that connects the two closest parts of the bend. This causes the flow to abandon the meander and to continue straight downslope. Cutoffs are a natural part of the evolution of a meandering river, and have also been used to artificially shorten the length of meandering rivers for navigation.
In March, 1876, a cutoff formed suddenly across the neck of a meander in the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

 near Reverie, Tennessee

Reverie, Tennessee

Reverie is an unincorporated community in Tipton County, Tennessee, United States. In 2001, the population was 11.Over a period of about 24 hours on March 7, 1876, the Mississippi River abandoned its former channel that coincided with the Tennessee-Arkansas border, and established a new channel...

, shortening the river's course and leaving the town connected to Arkansas

Arkansas

Arkansas is a state located in the southern region of the United States. Its name is an Algonquian name of the Quapaw Indians. Arkansas shares borders with six states , and its eastern border is largely defined by the Mississippi River...

but across the new river channel from the rest of Tennessee.