From AMS Glossary
A low-level mesoscale boundary or transition zone hundreds of kilometers in length and up to tens of kilometers in width separating dry air from moist air.
The length of the dryline is related to large-scale terrain or large-scale weather system features, whereas its width is related to mesoscale processes. In its quiescent state, the dryline may be considered the intersection of the top of a low-level moist layer with large-scale features of sloping terrain. In this state the shallow layer of moisture near the higher terrain is eroded by turbulent mixing with daytime heating. Moisture gradients are additionally strengthened by horizontal convergence resulting from downward transport of horizontal momentum in the dry air. In a more dynamically active state the dryline often advances away from the higher terrain as an integral component of an extratropical cyclone or frontal wave. In such cases it extends equatorward from the cyclone or wave. In this state moisture gradients and boundary motion are largely influenced by downward transport of horizontal momentum resulting from larger-scale sinking in the dry air. The dryline is found all over the world. In the United States the dryline, which marks the boundary between moist air from the Gulf of Mexico and dry continental air from the west, is found in the Plains region. It is most often present during the spring, where it is often the site of thunderstorm development. Typically the dryline in the United States advances eastward during the day and retreats westward at night.