Which desert plateau lies between the Caspian Sea and the Aral Sea?

The desert plateau between the Caspian Sea and the Aral Sea is the Ustyurt Plateau.

Ustyurt is a vast, elevated clay-desert region of Central Asia shared by Kazakhstan, Turkmenistan, and Uzbekistan. It lies west of the Aral Sea and east of the Caspian Sea, although its boundaries also touch features such as the Amu Darya delta and Sarygamysh Lake. The plateau averages roughly 150 metres above sea level and covers about 200,000 square kilometres.

Its most recognizable landforms are the steep escarpments known as chinks. These cliffs expose vividly layered rock and fall sharply toward surrounding lowlands, including the former western shoreline of the Aral Sea. The surface is mostly barren clay, gravel, and sparse desert vegetation rather than a field of shifting sand dunes.

A common mix-up is the Karakum Desert, which lies mainly in Turkmenistan south of the Ustyurt region. Ustyurt is also a geographic plateau and a desert, so the question's wording points to its full name rather than to a separate desert basin.