user-1: I have heard that when a meditator does a long meditation, he is able to enter into different world, see and hear that world. What are those? Are they just illusion, imagination or different universe? 

user11009: What is called world in Buddhism is the six senses. When you meditate you experience finer senses which you can't experience via ear, nose etc. When you meditate you experience finer wholesome mental states.
The logic here is for example you can see only colours from your eye. Even if you become a Deva or Brahama with your eye you only see colours. 

user8157: In Pali Buddhism, the word 'world' ('loka') refers to different mental states. 
For example, MN 79 calls the meditative jhanas of pure bliss 'the world of only pleasant feelings' ('ekantasukhassa lokassa'). To quote: 

Venerable sir, what is that course of actions to realise the world of only pleasant feelings?
Here, Udāyi, the bhikkhu secluded from sensual desires and thoughts of demerit abides in the first jhana: Overcoming thoughts and thought processs and the mind in one point internally appeased, without thoughts and thought processes abides in the second jhana. Again with equanimuity to joy and detachment, feeling pleasant with the body too, abides in the third jhana. To this the noble ones say abiding in pleasantness with equanimity. Udāyi, this is the course of actions, for realising the world of only pleasant feelings. `
MN 79

'Nibbana' is not a 'world'. The attainment of Nibbana is the 'cessation of the world', which is experienced within the living body & mind. To quote: 

There is, bhikkhus, that [sense] base where there is no earth, no water, no fire, no air; no base consisting of the infinity of space,
  no base consisting of the infinity of consciousness, no base
  consisting of nothingness, no base consisting of
  neither-perception-nor-non-perception; neither this world nor
  another world nor both; neither sun nor moon. Here, bhikkhus, I say
  there is no coming, no going, no staying, no deceasing, no uprising.
  Not fixed, not movable, it has no support. Just this is the end of
  suffering. Ud 8.1

Within this fathom-long body, with its perception & mind, that I declare that there is the world, the origination of the world, the
  cessation of the world and the path of practice leading to the
  cessation of the world. AN 4.45

Therefore, when a meditator does a long meditation & is able to enter into different world, those worlds are not just illusion, not imagination or not a different universe. Those worlds are mental states. 
As for Nibbana, this is also not illusion, not imagination & not a different universe. However, it is also not another world and not a mental state, even though Nibbana is experienced by the mind. 

-1: <p>I have heard that when a meditator does a long meditation, he is able to enter into different world, see and hear that world. What are those? Are they just illusion, imagination or different universe? </p>
 11009: <p>What is called world in Buddhism is the six senses. When you meditate you experience finer senses which you can't experience via ear, nose etc. When you meditate you experience finer wholesome mental states.
The logic here is for example you can see only colours from your eye. Even if you become a Deva or Brahama with your eye you only see colours. </p>
 8157: <p>In Pali Buddhism, the word 'world' ('loka') refers to different mental states. </p>

<p>For example, MN 79 calls the meditative jhanas of pure bliss '<em>the world of only pleasant feelings</em>' ('<em>ekantasukhassa lokassa</em>'). To quote: </p>

<blockquote>
  <p><em>Venerable sir, what is that course of actions to realise the world of only pleasant feelings?</em></p>
  
  <p><em>Here, Udāyi, the bhikkhu secluded from sensual desires and thoughts of demerit abides in the first jhana: Overcoming thoughts and thought processs and the mind in one point internally appeased, without thoughts and thought processes abides in the second jhana. Again with equanimuity to joy and detachment, feeling pleasant with the body too, abides in the third jhana. To this the noble ones say abiding in pleasantness with equanimity. Udāyi, this is the course of actions, for realising the world of only pleasant feelings. `</em></p>
  
  <p><a href="http://awake.kiev.ua/dhamma/tipitaka/2Sutta-Pitaka/2Majjhima-Nikaya/Majjhima2/079-culasakuludayi-e1.html" rel="nofollow noreferrer">MN 79</a></p>
</blockquote>

<p>'Nibbana' is not a 'world'. The attainment of Nibbana is the 'cessation of the world', which is experienced within the living body &amp; mind. To quote: </p>

<blockquote>
  <p><em>There is, bhikkhus, that [sense] base where there is no earth, no water, no fire, no air; no base consisting of the infinity of space,
  no base consisting of the infinity of consciousness, no base
  consisting of nothingness, no base consisting of
  neither-perception-nor-non-perception; <strong>neither this world nor
  another world nor both</strong>; neither sun nor moon. Here, bhikkhus, I say
  there is no coming, no going, no staying, no deceasing, no uprising.
  Not fixed, not movable, it has no support. Just this is the end of
  suffering. Ud 8.1</em></p>
  
  <hr>
  
  <p><strong><em>Within this fathom-long body, with its perception &amp; mind</strong>, that I declare that <strong>there is</strong> the world, the origination of the world, <strong>the
  cessation of the world</strong> and the path of practice leading to the
  cessation of the world. AN 4.45</em></p>
</blockquote>

<p>Therefore, when a meditator does a long meditation &amp; is able to enter into different world, those worlds are not just illusion, not imagination or not a different universe. Those worlds are mental states. </p>

<p>As for Nibbana, this is also not illusion, not imagination &amp; not a different universe. However, it is also not another world and not a mental state, even though Nibbana is experienced by the mind. </p>
