According to the dictionary meaning, revelation is the conveying of a message in a secret manner to another. In Islamic terminology it is defined as the dialogue of Almighty Allah with the prophets. They were claiming to have a special connection with God and that Almighty Allah speaks to them and conveys His messages to the people through them. They claimed that they can hear the statements of Almighty Allah and see realities in the unseen world and that they have been appointed on behalf of God so that they may convey His messages to the people.
Knowledge of the prophets is obtained through the channel of revelation and it is absolutely opposed to our acquisition of knowledge. We have three kinds of knowledge: Knowledge of senses, knowledge gained through generalities and internal perceptions and realizations. Knowledge of senses is obtained directly from the five senses. The senses also have a role in the knowledge of generalities, because their parts previously were realized through the channel of these senses. After that the generalities are separated from them. The third type is through the inner senses, like: the perception of pain, hunger, thirst, happiness and sorrow. All our knowledge is obtained through the channel of inner or outer senses directly or indirectly. But revelation is not a part of any of them and is having no similarity to them.
Prophets witness the reality in the unseen world and hear the words of God, but not with these eyes and ears. In this instance, knowledge is transferred from Almighty Allah to the heart of the prophets through revelation and at that moment it becomes apparent. Exactly opposed to it is ordinary knowledge which in the beginning is obtained through senses and in the end it enters our selves and hearts. The Quran has also interpreted revelation in this manner:
وَإِنَّهُ لَتَنزِیلُ رَبِّ الْعَالَمِینَ / نَزَلَ بِهِ الرُّوحُ الْأَمِینُ / عَلَى قَلْبِکَ لِتَکُونَ مِنَ الْمُنذِرِینَ / بِلِسَانٍ عَرَبِىٍّ مُّبِینٍ
“And most surely this is a revelation from the Lord of the worlds. The Faithful Spirit has descended with it. Upon your heart that you may be of the warners, In plain Arabic language.” (26:192-195)
قُلْ مَنْ کَانَ عَدُوّاً لِّجِبْرِیْلَ فإِنَّهُ نَزَّلَهُ عَلَى قَلْبِکَ
“Say: Whoever is the enemy of Jibraeel- for surely he revealed it to your heart…” (Surah Baqarah 2:97)
It is mentioned in Tafsir Ruhul Bayan that:
Whenever revelation descended on Prophet Muhammad (S), it first descended upon his heart as he was extremely thirsty for it. At that moment his heart absorbed it. Then he understood it through his heart and heard it and this is in the meaning of coming down from a height and this is the status of the special ones.
The late Allamah Tabatabai (q) has said: Heart denotes the self of man, which has the power of perception, perhaps the reason why Quran has mentioned:
نَزَلَ بِهِ الرُّوحُ الْأَمِینُ / عَلَى قَلْبِکَ
“The Faithful Spirit has descended with it. Upon your heart…” (26:193-194)
And did not say: “upon you”, which indicates the comprehensiveness of Quran from the side of the Holy Prophet (s) and also whatever he received from the soul was his noble self without his conscious perception having any interference in it.
Therefore, he hears whatever is revealed to him without the intervention of his eyes and ears. Because if his seeing and hearing had been through the apparent eyes and ears, everything he heard would be shared by others; that is whatever he sees, people also see it, while the fact is that traditional reports have refuted this meaning absolutely.
That is why, the knowledge of prophets, which is obtained through revelation, is not like obtained knowledge learned through perception and human thinking, on the contrary it is of a higher type, whose essence is not clear to us. It is a type of secret and hidden understanding and errors and doubts cannot enter it.
From some verses, it is learnt that revelation takes place in one of the following three ways:
First method: Almighty Allah reveals matters directly on the heart of the prophet.
Second method: Matters are sent down to another place and the prophet obtains from there, like in the case of the conversation of Prophet Musa (a) at Mount Tur through the tree. The Holy Quran says:
فَلَمَّآ أَتَاهَا نُودِىَ مِن شَاطِىِ الْوَادِ الْأَیْمَنِ فِى الْبُقْعَةِ الْمُبَارَکَةِ مِنَ الشَّجَرَةِ
“And when he came to it, a voice was uttered from the right side of the valley in the blessed spot of the bush…” (28:30)
Third method: Some points are revealed through the angel of revelation (Jibraeel) on the heart of the prophet.
These three methods are mentioned in the Holy Quran when Almighty Allah says:
وَمَا کَانَ لِبَشَرٍ أَن یُکَلِّمَهُ اللَّهُ إِلَّا وَحْیاً أَوْ مِن وَرَآىِ حِجَابٍ أَوْ یُرْسِلَ رَسُولًا فَیُوحِىَ بِإِذْنِهِ مَا یَشَآءُ إِنَّهُ عَلِىٌّ حَکِیمٌ
“And it is not for any mortal that Allah should speak to him except by revelation or from behind a veil, or by sending a messenger and revealing by His permission what He pleases, Surely He is High, wise.” (42:51)
But it should be known that in every method, the dialogue is only with Almighty Allah and that is why in most verses, revelation is related to Almighty Allah, but through a medium of cause and effects, one of them being Jibraeel.
Hence, these three methods of revelation should be interpreted in the effect of psychological conditions of the prophet and various divine emotions. In this case, sometimes the ethereal soul of the prophet rises up to the position of Jibraeel and he hears the divine revelation from him. But he does not see himself till he himself saw Jibraeel with his eyes. Sometimes the progress of his soul is to the limit that he hears the speech of Almighty Allah at a designated place like a tree etc. In that case also, his soul rises up such a level that he sees the medium and hears the dialogue directly from Almighty Allah.
Allamah Tabatabai (q) has mentioned the same point when he says:
The proof of veil (Hijab) or the conveyer of the message at the time of speaking does not use the medium of revelation, because revelation like his other characters is not without a medium, only the pivot is the attention of the one who is addressed. Thus if the prophet were to see through which he is getting divine revelation, and through this medium he veils the statement and message, which has come to him from Almighty Allah; like the medium of angel, in that case revelation is on that angel. Thus when the prophet looks at God, the revelation will be from Almighty Allah. Although there is also a medium, but the prophet pays no attention to it.
That is why revelation is a kind of a hidden and inner understanding and perception, which the apparent and hidden perceptions and logical prefaces have no intervention in. On the contrary, it is directly inspired to the heart of the prophet. Since the self of the prophet has reached to the highest level of humanity it has the capacity to obtain such information; but revelation is not ordinary knowledge; it is an extraordinary matter. Therefore people demand miracle from the prophets. The latter also in order to prove the veracity of their claim, perform miracles in situations that demand it.
 Tafsir Ruhul Bayan, Vol. 6, Pg. 306
 Al-Mizan, Vol. 15, Pg. 345
 Al-Mizan, Vol. 14, Pg. 150