This is a unique word used only in the Bhagavadgītā and that too, only three times (2.49; 10.10 and 18.57).
‘Buddhi’ means the intellect and ‘yoga’ means perfect concentration as well as union. So, the path of spiritual discipline wherein the buddhi plays an important role and ultimately leads to the union of the jīva (the individual soul) with Paramātman (God) is buddhiyoga.
Karma or action normally binds a person due to the selfish desires behind it. If karma is performed with the full understanding that its motivation by selfish desires leads to saṁsāra or the bondage of transmigration, and that, if motivated by the spirit of unselfish service and devotion to God, the same will lead to liberation, it becomes buddhiyoga since this leads to the ultimate union with God.
The commentators of the Bhagavad-gītā have interpreted it variously, depending upon the context. However, all the interpretations boil down to the same meaning as given above.
These are some of them: karma per-formed with equanimity; niṣkāma-karma-yoga that leads to ātmajñāna or self-know-ledge; yoga endowed with the vision of the ātman; uniting the mind with God and so on.
See also BHAGAVADGĪTĀ.