Kautilya's Arthashastra 6.2.22

6.2.22

arivijigīṣumadhyānāṃ bahiḥ prakṛtibhyo balavattaraḥ saṃhatāsaṃhatānām arivijigīṣumadhyamānām anugrahasamartho nigrahe cāsaṃhatānām udāsīnaḥ //

He who is situated beyond the territory of any of the above kings and who is very powerful and capable of helping the enemy, the conqueror, and the Madhyama king together or individually, or of resisting any of them individually, is a neutral king (udásína),--these are the (twelve) primary kings.