SB 7.8.17

satyaṁ vidhātuṁ nija-bhṛtya-bhāṣitaṁ
 vyāptiṁ ca bhūteṣv akhileṣu cātmanaḥ
adṛśyatātyadbhuta-rūpam udvahan
 stambhe sabhāyāṁ na mṛgaṁ na mānuṣam
Word for word: 
satyam — true; vidhātum — to prove; nija-bhṛtya-bhāṣitam — the words of His own servant (Prahlāda Mahārāja, who had said that his Lord is present everywhere); vyāptim — the pervasion; ca — and; bhūteṣu — among the living entities and elements; akhileṣu — all; ca — also; ātmanaḥ — of Himself; adṛśyata — was seen; ati — very; adbhuta — wonderful; rūpam — form; udvahan — taking; stambhe — in the pillar; sabhāyām — within the assembly; na — not; mṛgam — an animal; na — nor; mānuṣam — a human being.
Translation: 
To prove that the statement of His servant Prahlāda Mahārāja was substantial — in other words, to prove that the Supreme Lord is present everywhere, even within the pillar of an assembly hall — the Supreme Personality of Godhead, Hari, exhibited a wonderful form never before seen. The form was neither that of a man nor that of a lion. Thus the Lord appeared in His wonderful form in the assembly hall.
Purport: 

When Hiraṇyakaśipu asked Prahlāda Mahārāja, “Where is your Lord? Is He present in this pillar?” Prahlāda Mahārāja fearlessly replied, “Yes, my Lord is present everywhere.” Therefore, to convince Hiraṇyakaśipu that the statement of Prahlāda Mahārāja was unmistakably true, the Lord appeared from the pillar. The Lord appeared as half lion and half man so that Hiraṇyakaśipu could not understand whether the great giant was a lion or a human being. To substantiate Prahlāda’s statement, the Lord proved that His devotee, as declared in Bhagavad-gītā, is never vanquished (kaunteya pratijānīhi na me bhaktaḥ praṇaśyati). Prahlāda Mahārāja’s demoniac father had repeatedly threatened to kill Prahlāda, but Prahlāda was confident that he could not be killed, since he was protected by the Supreme Lord. By appearing from the pillar, the Lord encouraged His devotee, saying in effect, “Don’t worry. I am present here.” By manifesting His form as Nṛsiṁhadeva, the Lord also preserved the truth of Lord Brahmā’s promise that Hiraṇyakaśipu was not to be killed by any animal or any man. The Lord appeared in a form that could not be said to be fully a man or a lion.