Upanishads (उपनिषद्) are part of the Hindu Shruti scriptures which primarily discuss meditation and philosophy and are seen as religious instructions by most schools of Hinduism.
The Upanishads are commentaries on the Vedas, their putative end and essence, and thus known as Vedanta ("End of the Veda").
Of the early Upanishads, the Aiterya and Kaushitaki belong to the Rig Veda, Kena and Chandogya to the Sama, Isha and Taittereya and Brihadaranyaka to the Yajur, and Prasna and Mundaka to the Atharva.
The Upanishads were composed over several centuries, the oldest such as the Brhadaranyaka and Chandogya upanisads have been dated to around the eighteenth century BCE and the later ones to as late as the fifteenth century BCE.
The oldest Upanishads, the Bṛhadāraṇyaka and the Chāndogya are composed in prose.
Of the early Upanishads, the Aitareya and Kauṣītāki belong to the Rig Veda, Kena and Chāndogya to the Samaveda, Īṣa and Taittirīya and Bṛhadāraṇyaka to the Yajurveda, and Praṣna and Muṇd.aka to the Atharvaveda.