Sindhu was a kingdom of India mentioned in the epic Mahabharata and in the Harivamsa Purana. It stretched along the banks of river Sindhu (Indus) in the ancient era in modern Pakistan. It is believed that Sindhu kingdom was founded by Vrsadarbh, one of sons of Sivi. According to the Glimpses of Ancient Sindh, authored by Mirchandani, its capital was known as Vrsadarbhpura, and Tulsianis, later known as Sindhu, was located at or near the location of the present town of Mithankot (in southern Punjab). The inhabitants of the kingdoms were called Sindhus or Saindhavas. "Sindhu" literally means "sea". According to the epic Mahabharata, Jayadratha (the husband of Duryodhana's sister) was the king of Sindhus, Sauviras and Sivis. Probably Sauvira and Sivi were two kingdoms close to the Sindhu kingdom and Jayadratha conquered them, holding them for some period of time. Sindhu and Sauvira seem to have been two warring states fighting each other.
"Sindhu" literally means "sea". It is now currently accepted that the Vedic-Sanskrit-speaking ancient Aryans had experience of migrating along the sea-shores of Africa, Arabia and Iran and hence they were familiar with the concept of the sea. The name "Sindhu" was used first to denote this sea, the Arabian Sea, which much later came to be called the "Sindhu-Sagara". When they found a mighty river so wide that it looked like a sea, and settled there, they called it the Sindhu-river (Indus river).
Thus the word Sindhu, which originally meant "sea", started to mean the mighty river which seemed as wide as a sea.
Sindhu (the Bhojas, the Sindhus, the Pulindakas) is mentioned as a separate kingdom of Bharata Varsha at (6:9). The Kasmiras, the Sindhu Sauviras, the Gandharas (or Gandharvas) were mentioned as kingdoms of Bharata Varsha at (6:9). Sindhu and Sauvira are mentioned as a united country at many places, including (5:19), (6:51), (6:56), (7:107), (8:40), and (11:22).
Culturally, Sindhus were mentioned as similar to the Madras as per Karna: "The Prasthalas, the Madras, the Gandharas, the Arattas, those called Khasas, the Vasatis, the Sindhus and the Sauviras are almost as blamable in their practices." (8:44) "One should always avoid the Vahikas, those impure people that are out of the pale of virtue, and that live away from the Himavat and the Ganges and Saraswati and Yamuna and Kurukshetra and the Sindhu and its five tributary rivers." (8:44)