SB 10.8.12

śrī-garga uvāca
ayaṁ hi rohiṇī-putro
 ramayan suhṛdo guṇaiḥ
ākhyāsyate rāma iti
 balādhikyād balaṁ viduḥ
yadūnām apṛthag-bhāvāt
 saṅkarṣaṇam uśanty api
Synonyms: 
śrī-gargaḥ uvāca — Garga Muni said; ayam — this; hi — indeed; rohiṇī-putraḥ — the son of Rohiṇī; ramayan — pleasing; suhṛdaḥ — all His friends and relatives; guṇaiḥ — by transcendental qualities; ākhyāsyate — will be called; rāmaḥ — by the name Rāma, the supreme enjoyer; iti — in this way; bala-ādhikyāt — because of extraordinary strength; balam viduḥ — will be known as Balarāma; yadūnām — of the Yadu dynasty; apṛthak-bhāvāt — because of not being separated from you; saṅkarṣaṇam — by the name Saṅkarṣaṇa, or uniting two families; uśanti — attracts; api — also.
Translation: 
Garga Muni said: This child, the son of Rohiṇī, will give all happiness to His relatives and friends by His transcendental qualities. Therefore He will be known as Rāma. And because He will manifest extraordinary bodily strength, He will also be known as Bala. Moreover, because He unites two families — Vasudeva’s family and the family of Nanda Mahārāja — He will be known as Saṅkarṣaṇa.
Purport: 

Baladeva was actually the son of Devakī, but He was transferred from Devakī’s womb to that of Rohiṇī. This fact was not disclosed. According to a statement in the Hari-vaṁśa:

pratyuvāca tato rāmaḥ
 sarvāṁs tān abhitaḥ sthitān
yādaveṣv api sarveṣu
 bhavanto mama vallabhāḥ

Garga Muni did disclose to Nanda Mahārāja that Balarāma would be known as Saṅkarṣaṇa because of uniting two families — the yadu-vaṁśa and the vaṁśa of Nanda Mahārāja — one of which was known as kṣatriya and the other as vaiśya. Both families had the same original forefather, the only difference being that Nanda Mahārāja was born of a vaiśya wife whereas Vasudeva was born of a kṣatriya wife. Later, Nanda Mahārāja married a vaiśya wife, and Vasudeva married a kṣatriya wife. So although the families of Nanda Mahārāja and Vasudeva both came from the same father, they were divided as kṣatriya and vaiśya. Now Baladeva united them, and therefore He was known as Saṅkarṣaṇa.