Ten Principal Upanishads known as 'Dashopanishad' are :Isha, Kena, Katha , Prashna , Munda , Mandukya, Taittiriya, Aitareya, Chandogya and Brihadaranyaka. Besides, Shvetashvatara, Kaushitaki and Maitrayaniya Upanishads are often listed in old Upanishads.
Isavasya Upanishad is a small Upanishad consisting of 18 mantras and it belongs to Shukla Yajur Veda
18 mantras of Isavasya Upanishad are elaborated in more than 400 mantras of Brihadaranyaka. From that we know that Isavasya is a very condensed Upanishad
This Upanishad gets it name because of the first portion of the beginning verse “Isavasyam Idam Sarvam”. The first two words are “Isa” and “Avasyam” and hence its called Isavasya. The first word is “Isa” so it can also be called Isa Upanishad or Isopanishad
The Upanishads are ancient Hindu philosophical and spiritual texts that mark the culmination of the Vedic literature and focus on meditation, consciousness, and the ultimate nature of reality and self. They are considered foundational works in Hindu thought, representing the last phase of Vedic revelation and exploring the knowledge of Brahman (the universal reality) and Atman (the individual soul).