In the Bhakti-rasāmṛta-sindhu, the following definition is given:

athāsyāḥ keśava-rater
lakṣitāyā nigadyate
sāmagrī-paripoṣena
paramā rasa-rūpatā

vibhāvair anubhāvaiś ca
sāttvikair vyabhicāribhiḥ
svādyatvaṁ hṛdi bhaktānām
ānītā śravaṇādibhiḥ
eṣā kṛṣṇa-ratiḥ sthāyī
bhāvo bhakti-raso bhavet

Love for Kṛṣṇa, Keśava, as previously described, reaches the supreme state of being composed of mellows when its ingredients are fulfilled. By means of vibhāva, anubhāva, sāttvika and vyabhicārī, hearing and chanting are activated, and the devotee is able to taste love for Kṛṣṇa. Then attachment for Kṛṣṇa, or permanent ecstasy (sthāyi-bhāva), becomes the mellow of devotional service (bhakti-rasa).