Manava Dharmashastra 2.21

2.21

uttarasyāṃ diśi himavānparvato dakṣiṇasyāṃ vindhyaḥ | vinaśanaṃ sarasvatyā antardhānadeśaḥ | prayāgo gaṅgāyamunayoḥ saṅgamaḥ | etāndeśānavadhīkṛtya madhyaṃ madhyadeśanāmānaṃ deśaṃ vidyāt | nātyutkṛṣṭo nātinikṛṣṭa ityato'yaṃ madhyadeśo, na tu pṛthivīmadhyabhavatvāt |

The country lying between the Himālaya and the Vindhya, to the east of Vinaśana and to the west of Prayāga, is called the ‘Madhyadeśa,’ the ‘Middle Country.’ (21)

Medhātithi’s commentary (manubhāṣya):
On the north lies the Himālaya and on the south the Vindhya. ‘Vinaśana’ is the name of the place where the Sarasvatī river has disappeared.—(20)

‘Prayāga’—is the confluence of the Gaṅgā and the Yamunā.

The region having these four as its boundaries is to be known by the name ‘Madhya-deśa.’ It is called ‘madhya’ or ‘middle,’ because it is neither very superior nor very inferior,—and not because it is located the centre of the Earth.—(21)