Põhigrupiks
Põhigrupiks is an Estonian term formed with the translative case (-iks) from the noun põhigrupp, meaning base group or core group. The translative case marks designation or a change into a role or state, so põhigrupiks is used to indicate that something is being described as the base group in a larger set, plan, or analysis.
In practice, põhigrupiks appears in academic and applied contexts to designate the main reference or anchor
Notes and variations: põhigrupiks is not a universally fixed technical term; different fields might prefer different