Definition of "bhakti"

It should be obvious by now that bhakti is not merely pious thoughts of "love" but rather fearless action. Nārada asks nothing less of the bhakta than complete surrender and complete dedication unto the will of Bhagavān. But if at any point one feels himself unable to reach the ideals taught by Nārada, he is not condemned. Lord Kṛṣṇa also says that if we cannot achieve the topmost surrender, then we should do what we can and try to progress gradually.

1991, A. C. Bhaktivedanta Swami Prabhupāda, Nārada-bhakti-sūtra, The Secrets of Transcendental Love, page 114

The offering of Bhakti is a specific activity, either mental or physical, through which a devotee can finally attain the knowledge of the Supreme Reality. So, Karma and Jñāna are not antagonistic to the Bhakti Mārga, rather, they are indispensable parts of it.

2004, Muhammad Sirajul Islam, Sufism and Bhakti, A Comparative Study, page 78