Passing by the windows of Srila Prabhupada·s house in Vrindavana, we saw inside a group of devotees conversing. Brahmananda Prabhu, one of the first devotees made by Srila Prabhupada in the 60's, had seemingly taken the lead of the discussion. As he always used to say intelligent and humorous things, we entered the room and sat down. They were reading the Bhagavad-gita, verse 9.4

maya tatam idam sarvam
jagad avyakta-murtina
mat-sthani sarva-bhutani
na caham tesv avasthitah

By Me, in My unmanifested form, this entire universe is pervaded. All beings are in Me, but I am not in them.

A brahmacari was giving the class and Brahmananda Prabhu challenged him to say which was the strangest things in this verse. He could not detect it and Brahmananda said:

'Murtinah' Unmanifested form. What is an unmanifested form? Can you say what is an unmanifested form?

The onlookers did not answer, some smiled at the humorous way he was puzzling about the philosophical concept of 'unmanifested form'.
How can a form being unmanifested? It seemed a contradiction in terms. Then he said:

'The only explanation is acintya-bhedabheda tattva.'

I did not intervene in the discussion, but the day after I went to read the verse and I did not find too much to be puzzled about. The verse appears to be clear enough. Let's see it.

'By Me, in My unmanifested form (avyakta-murtinah), this entire universe is pervaded. All beings are in Me, but I am not in them.'

Here Sri Krishna is saying that material universe is pervaded by His unmanifested form. Srila Prabhupada explains the concept with his usual clarity. The original form of the Lord is not perceivable by material senses; therefore His transcendental form is not visible to us. Being invisible, from our point of view, that form can be called unmanifested.

But although we cannot see Him, still the universe is pervaded by His potencies. His presence does not depend on our perception. We can see Him or not, still He is there. His potencies are Him and His, even though we cannot perceive this truth. The material energy is one of His energies, and in this phenomenal plane many other divine energies interact with us, like the impersonal energy and various incarnations. All these energies are different aspects of the Lord. Being partial manifestations, all these do not manifest the real and complete form of the Lord, and therefore it is correct to say that those are avyakta-murtinah (unmanifested forms) of the Lord.