SB 8.18.12

yat tad vapur bhāti vibhūṣaṇāyudhair
 avyakta-cid-vyaktam adhārayad dhariḥ
babhūva tenaiva sa vāmano vaṭuḥ
 sampaśyator divya-gatir yathā naṭaḥ
Synonyms: 
yat — which; tat — that; vapuḥ — transcendental body; bhāti — manifests; vibhūṣaṇa — with regular ornaments; āyudhaiḥ — and with weapons; avyakta — unmanifested; cit-vyaktam — spiritually manifested; adhārayat — assumed; hariḥ — the Lord; babhūva — immediately became; tena — with that; eva — certainly; saḥ — He (the Lord); vāmanaḥ — dwarf; vaṭuḥ — a brāhmaṇa brahmacārī; sampaśyatoḥ — while both His father and mother were seeing; divya-gatiḥ — whose movements are wonderful; yathā — as; naṭaḥ — a theatrical actor.
Translation: 
The Lord appeared in His original form, with ornaments and weapons in His hands. Although this ever-existing form is not visible in the material world, He nonetheless appeared in this form. Then, in the presence of His father and mother, He assumed the form of Vāmana, a brāhmaṇa-dwarf, a brahmacārī, just like a theatrical actor.
Purport: 

The word naṭaḥ is significant. An actor changes dress to play different parts, but is always the same man. Similarly, as described in the Brahma-saṁhitā (5.33, 39), the Lord assumes many thousands and millions of forms (advaitam acyutam anādim ananta-rūpam ādyaṁ purāṇa-puruṣam). He is always present with innumerable incarnations (rāmādi-mūrtiṣu kalā-niyamena tiṣṭhan nānāvatāram akarod bhuvaneṣu kintu). Nonetheless, although He appears in various incarnations, they are not different from one another. He is the same person, with the same potency, the same eternity and the same spiritual existence, but He can simultaneously assume various forms. When Vāmanadeva appeared from the womb of His mother, He appeared in the form of Nārāyaṇa, with four hands equipped with the necessary symbolic weapons, and then immediately transformed Himself into a brahmacārī (vaṭu). This means that His body is not material. One who thinks that the Supreme Lord assumes a material body is not intelligent. He has to learn more about the Lord’s position. As confirmed in Bhagavad-gītā (4.9), janma karma ca me divyam evaṁ yo vetti tattvataḥ. One has to understand the transcendental appearance of the Lord in His original transcendental body (sac-cid-ānanda-vigraha).