"Upanishad" is made up of two words.
“Sub” us “Nishad”.
The sub-word means "near" or "near" and "nishad means" sitter.
Thus, the Upanishads literally mean "sitting at the feet of Guru or Paramatman and attaining mystery or mysticism."
According to some critics, "Upanishads are the scriptures that destroy ignorance and move towards paramatman or knowledge."
It is also called "Vedanta" or "The End of Vedas" as the Upanishads were composed at the end of Vedic literature.
According to scholars, Upanishads have been formed for a long time. Initially it was oral and over time it was transcribed.
The number of Upanishads is about $108,$ the names of the main Upanishads are as follows.
$(1)$ Ishopanishad $(2)$ Prashnopanishad $(3)$ Taitriya Upanishad $(4)$ Nrusinhapurvatap Upanishad $(5)$ Canopanishad $(6)$ Mundkopanishad $(7)$ Aitarey Upanishad $(8)$ Brihadaranyak Upanishad $(9)$ Kathopanishad