The closest description of Brahman is Satchidananda which literally translates as Existence Knowledge Bliss Absolute. But even that is only describing the outer layer of Brahman so to speak as Brahman is beyond all words. All you can say of Brahman is "neti, neti" - not this, not this (meaning whatever you can put into words or experience cannot be Brahman) Sat is translated as Existence, not 'Being'. Sri Krishna says "It is the Supreme Brahman, which is without beginning and is said to be neither being nor non-being." (Bhagavad Gita, Chapter XIII, verse 12, Swami Nikhilananda translator). All you can say of Brahman is - Brahman IS. After 40 years of study and numerous readings and studies, I have never seenthe most widely accepted translation of Sat translated as Beingis Existence. I have only seen SatIt can be translated as Existencebeing, but the use of the word being in English can lead to more mis-interpretations than the word existence.
As far as a sanskrit term that might approximate letheia, it may be 'maya'. Maya is ignornace obscuring the vision of God (Brahman); the Cosmic Illusion on account of which the One appears as many, the Absolute as the Relative. Maya is that which covers Brahman and makes us forget our true nature as Brahman.