It is-
•Eternal= Nityaha
•The Reality= Sathyaha
•All pervading= Sarvagathaha
•Never an object; ever the subject= Aprameya
•Not the doer; not the enjoyer= Akarta; Abhokta
•Does not change= Nirvikara
Answer= Atma.
Or call it by whatever name. We need a term to identify that One, which satisfies all the above.
Synonyms: Brahman, Aum