Question

why it doesn't use the word federal?

Answer 1

as because federation is a unit which is fully based on the principle of federalism but in india there is also some features of unitary government that's why we have not used federation for it

Answer 2

Union: It is there least federal structure. India is an indestructible union of destructible units (i.e. states can be destroyed or created by mergers, separation, changing the boundary, etc by the Union but they can't claim sovereignty and get separated from rest of the Union).Federation: The units are indestructible and so is the Union (or federation, whatever you want to call it). In USA states can't be tempered with, though they can't leave the federatiBut one must note that even these definitions are subjective and I've given the interpretation according to Indian point of view, for multiple nations use the word Union for different purposes (e.g. the Constitution of erstwhile USSR also used this term which was actually confederation).

The other thing to note is that India is not quasi-federal (which would mean that it only looks federal but it's actually not), she's federal. Yes the degree of federalism varies across the world and we've adopted a lesser degree according to our needs.

Third the essence of federalism lies in Division of Power (i.e. states have their own legislative and executive powers unfettered by the Union interventions). I didn't delve into those details for the question pertained to difference between union and federation and not to the definition of federalism.on either.