What is its etymology or definition of Veda?
The word "veda" just means knowledge. It comes from the root word "vid" (in Sanskrit), which means "to know"/"knowledge"/"know", etc. It is also a stem of the verb "vedayati" which means "cause to know". In other words, the Vedas cause people to know spiritual facts by conveying them through words.
What is a criteria by which a particular revelation can be considered a part of Veda?
This is a good question. There are spiritual revelations in the Bhagavad Gita, Itihasas, Puranas, Agamas etc. However, they aren't part of the Vedic texts. So what is the difference?
The Mimamsa definition is that the Vedic verses are eternal and unauthored, and verses found in Smriti are authored. In other words, verses that are eternal and unauthored are called shruti/vedas whereas authored verses are called agama/smriti.
However this definition is refuted by the Vedas themselves where the Rishis say they created the mantras:
RV 1.61.4: asmā idu stomaṃ saṃ hinomi rathaṃ na taṣṭeva — “For him, I
design this hymn, just as a carpenter designs a chariot”
Also, one of the oldest Vedic scholars, Yaska, says in his Nirukta:
Nirukta 1.20 - The Rishis were those who had a direction perception of Dharma. They gave mantras through instruction for those inferior people who do not have a direct perception of Dharma....[they are] the Vedas and Vedangas.
So, it appears that:
The Vedas are composed by Rishis based on a direct perception of "Dharma" (which is Brahman). So, the Rishis directly observe God, and then compose the Vedas for the sake of people who do not have that vision.
The Vedas are older than the Smritis. The Vedic language is older than the language of the Smritis. The Vedic language had a pitch accent which was gone by the time of Panini who simplified the language and hence non-existent in the Smritis.
The Vedas have their own meter (Chandas) and classification into rik, yajus, saman, and mantra (samhita), brahmana, aranyaka, and upanishad. This classification is not found in the Smritis.
Smritis always reference the authority of the Vedas. Vedas are more authoritative than Smritis. Since the Smritis come after the Vedas, the authors of the Smritis always reference the authority of the Vedas. In the Mahabharata, the speakers reference the authority and knowledge in the Vedas. In the Bhagavad Gita, Krishna says that what he says matches the knowledge in the Vedas. The Pancharatra Agamas, which were composed by Vishnu, are a simplification of the meaning of the lost Ekayana shakha of the Shukla Yajur Veda. References to the Smritis in the Vedas are very limited, and mainly occur in the Upanishads.
The Vedas also have varna restrictions. Only Dvijas who have done Upanayanam are eligible to recite Vedas. Agamas, etc. do not have varna restrictions.
So, the Vedas are the name given to a large body of peculiar literature composed a very long time ago.
It's quite well-known fact that the word Veda connotes Knowledge, but of what? All subject fields are knowledge only. What field isn't related to knowledge?
This is why the the Mahabharata, Itihasas, Puranas, etc. are termed the "5th Veda", because the knowledge they contain is equivalent to what is found in the Vedas even though they are not Vedas proper since they don't meet the criteria I listed above.