Transoxiana is the historical region of Central Asia lying roughly between the Amu Darya and Syr Darya rivers, broadly modern Uzbekistan and Tajikistan, whose oasis cities Samarkand and Bukhara made it a Silk Road crossroads and, after the Arab conquest, a hearth of Persianate-Islamic culture and science.
To the Arab armies who crossed the Oxus in the eighth century, this was Mā warāʼ al-nahr, simply 'the land beyond the river,' a frontier named from the far bank. Within two hundred years the frontier had outgrown the vantage that named it: Bukhara's libraries and Samarkand's workshops rivaled Baghdad itself.