Bhakti (भक्ति, Bhakti)
The term Bhakti, which means devotion, is derived from the Sanskrit root bhaj (भज्, bhaj) which means seva (सेवा, sevā) or meditation. In a common parlance, the word Bhakti is understood in the sense of love towards the respected deity or any elderly person on whom there is respect. Love is actually a state of knowledge, a mental disposition. In the case of Bhakti with reference to a God or a chosen deity, it means unceasing meditation with intense love for the Supreme being. It is interesting to note that this general concept of Bhakti in the sense of devotion or reverence to a deity or God (with or without form) is more or less accepted by all religions (1).
Bhakti and Bhakti-yoga:
When the seemingly general bhakti is practised rigorously and continuously for the life-time as a religio-spiritual discipline by a qualified aspirant * until the attainment of the final end namely the complete liberation from bondage, it is termed as Bhakti-yoga as outlined in the Bhagavad-gita. In the Upanishads, the word bhakti does not find a direct mention but instead referred to as upasana (उपासना, upāsanā). The bhakti-yoga or upasana of Brahman (for the purpose of liberation) does not merely involve the bhakti in the general sense but the entire ashtanga-yoga or the eight-fold ethico-religious discipline as explained in the Yoga system. Infact when Bhakti-yoga is pursued to attain liberation, it should be preceded by the acquisition of the philosophic knowledge about the individual soul, the Brahman and the manifested universe. Thus there is a wide difference between the terms bhakti and bhakti-yoga (2).
References and Footnotes:
(1) Vaisnavism, by S.M.Srinivasa Chari, page 99-100.
* According to the traditional Vedantic perspective, not everyone can embark upon the path of Bhakti-yoga and there are some eligibility criteria.
(2) Vaisnavism, by S.M.Srinivasa Chari, page 100