Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura quotes the following definition of rasābhāsa from the Bhakti-rasāmṛta-sindhu (Uttara-vibhāga, Ninth Wave, 1-3, 33, 38 and 41):

pūrvam evānuśiṣṭena vikalā rasa-lakṣaṇā
rasā eva rasābhāsā rasajñair anukīrtitāḥ

syus tridhoparasāś cānu- rasāś cāparasāś ca te
uttamā madhyamāḥ proktāḥ kaniṣṭhāś cety amī kramāt

prāptaiḥ sthāyi-vibhāvānu- bhāvādyais tu virūpatām
śāntādayo rasā eva dvādaśoparasā matāḥ

bhaktādibhir vibhāvādyaiḥ kṛṣṇa-sambandha-varjitaiḥ
rasā hāsyādayaḥ sapta śāntaś cānurasā matāḥ

kṛṣṇa-tat-pratipakṣaś ced viṣayāśrayatāṁ gatāḥ
hāsādīnāṁ tadā te ’tra prājñair aparasā matāḥ

bhāvāḥ sarve tadābhāsā rasābhāsāś ca kecana
amī prokta-rasābhijñaiḥ sarve ’pi rasanād rasāḥ

“A mellow temporarily appearing transcendental but contradicting mellows previously stated and lacking some of a mellow’s necessities is called rasābhāsa, an overlapping mellow, by advanced devotees who know how to taste transcendental mellows. Such mellows are called uparasa (submellows), anurasa (imitation transcendental mellows) and aparasa (opposing transcendental mellows). Thus the overlapping of transcendental mellows is described as being first grade, second grade or third grade. When the twelve mellows — such as neutrality, servitorship and friendship — are characterized by adverse sthāyi-bhāva, vibhāva and anubhāva ecstasies, they are known as uparasa, submellows. When the seven indirect transcendental mellows and the dried-up mellow of neutrality are produced by devotees and moods not directly related to Kṛṣṇa and devotional service in ecstatic love, they are described as anurasa, imitation mellows. If Kṛṣṇa and the enemies who harbor feelings of opposition toward Him are respectively the object and abodes of the mellow of laughter, the resulting feelings are called aparasa, opposing mellows. Experts in distinguishing one mellow from another sometimes accept some overlapping transcendental mellows (rasābhāsa) as rasas due to their being pleasurable and tasteful.” Śrīla Viśvanātha Cakravartī Ṭhākura says, paraspara-vairayor yadi yogas tadā rasābhāsaḥ: “When two opposing transcendental mellows overlap, they produce rasābhāsa, or an overlapping of transcendental mellows.”