Tathāgata (Devanagari: तथागत) in Pali and Sanskrit (Chin., Jpn.: 如来; Kor.:여래; Vietnamese: Như Lai; Tibetan དེ་བཞིན་གཤེགས་པ) is the name the historical Buddha used when referring to himself while he was alive. Literally, it means both one who has thus gone (Tathā-gata) and one who has thus come (Tathā-āgata). Hence, the Tathagatas are beyond all coming and going. It is asserted by some that the name really means one who has found the truth. 
The Buddha of the scriptures is always reported as referring to himself by "the Tathagata" instead of using pronouns (me, I, myself). This serves to emphasize by implication that the words are uttered by one who has transcended the human condition, and is beyond the otherwise endless cycle of rebirth, beyond all death and dying, beyond all suffering.
The word is also used as a synonym for arahant.:227 It refers to someone who has attained the highest goal of the religious life: "a tathāgata, a superman (uttama-puriso)".:228 In Buddhist thought, such an individual is no longer human.
Etymology & interpretation
Sanskrit grammar offers two possibilities for breaking up the compound: either tathā and āgata or tathā and gata.:381-382 Tathā means thus in Sanskrit and Pali, and Buddhist thought takes this to refer to what is called reality as-it-is (yathā-bhūta). This reality is also referred to as thusness or suchness (tathatā) indicating simply that it (reality) is what it is.
A Buddha or Arhat is defined as someone who 'knows and sees reality as-it-is' (yathā bhūta ñāna dassana). Gata 'gone' is the past passive participle of the verbal root gam 'go, travel'. Āgata 'come' is the past passive participle of the verb meaning 'come, arrive'.
Thus in this interpretation Tathāgata means literally either, “The one who has gone to suchness” or, "The one who has arrived at suchness".
A number of passages affirm that a tathāgata, or arahant, is "immeasurable", "inscrutable", "hard to fathom", and "not apprehended".:227 A tathāgata has abandoned that clinging to the skandhas (personality factors) that render citta (the mind) a bounded, measurable entity, and is instead "freed from being reckoned by" all or any of them, even in life. The aggregates of form, feeling, perception, mental formations, and cognizance that comprise personal identity have been seen to be dukkha (a burden), and an enlightened individual is one with "burden dropped".:229.The Buddha explains that "that for which a monk has a latent tendency, by that is he reckoned, what he does not have a latent tendency for, by that is he not reckoned.:227, SN 3.3.5. These tendencies are ways in which the mind becomes involved in and clings to conditioned phenomena. Without them, an enlightened person cannot be "reckoned" or "named"; he or she is beyond the range of other beings, and cannot be "found" by them, even by gods, or Mara.:230 In one passage, Sariputta states that the mind of the Buddha cannot be "encompassed" even by him.:416-417
The Buddha and Sariputta, in similar passages, when confronted with speculation as to the status of an arahant after death, bring their interlocutors to admit that they cannot even apprehend an arahant that is alive.:235 As Sariputta puts it, his questioner Yamaka "can't pin down the Tathagata as a truth or reality even in the present life." These passages imply that condition of the arahant, both before and after parinirvana, lies beyond the domain where the descriptive powers of ordinary language are at home; that is, the world of the skandhas and the greed, hatred, and delusion that are "blown out" with nirvana.:226
In the Aggi-Vacchagotta Sutta, an ascetic named Vaccha questions the Buddha on a variety of metaphysical issues. When Vaccha asks about the status of an arahant after death, the Buddha asks him in which direction a fire goes when it has gone out. Vaccha replies that the question "does not fit the case ... For the fire that depended on fuel ... when that fuel has all gone, and it can get no other, being thus without nutriment, it is said to be extinct." The Buddha then explains: "In exactly the same way ..., all form by which one could predicate the existence of the saint, all that form has been abandoned, uprooted, pulled out of the ground like a palmyra-tree, and become non-existent and not liable to spring up again in the future. The saint ... who has been released from what is styled form is deep, immeasurable, unfathomable, like the mighty ocean.":225 The same is then said of the other aggregates. A variety of similar passages make it clear that the metaphor "gone out, he cannot be defined (atthangato so na pamanam eti) refers equally to liberation in life.:91, 95 In the Aggi-Vacchagotta Sutta itself, it is clear that the Buddha is the subject of the metaphor, and the Buddha has already "uprooted" or "annihilated" the five aggregates.:95 In Sn 1074, it is stated that the sage cannot be "reckoned" because he is freed from the category "name" or, more generally, concepts. The absence of this precludes the possibility of reckoning or articulating a state of affairs; "name" here refers to the concepts or apperceptions that make propositions possible.:94
A similar response citing immeasurability occurs in another sutta, when the Buddha is asked to pick between two alternatives regarding the arahant after death: annihilation or eternal freedom from illness. The Buddha responds: "There is no measure of him who had achieved the goal. That by which one could define him, that is not for him. When all phenomena (dhamma) are removed, then all means of description are also removed.":225
Nagarjuna expressed this understanding in the nirvana chapter of his Mulamadhyamakakarika: "It is not assumed that the Blessed One exists after death. Neither is it assumed that he does not exist, or both, or neither. It is not assumed that even a living Blessed One exists. Neither is it assumed that he does not exist, or both, or neither.":230
- Nyanatiloka, Buddhist Dictionary, 4th ed. Buddhist Publication Society, 1980.
- Peter Harvey, The Selfless Mind. Curzon Press 1995
- Peter Harvey, An Introduction to Buddhism: Teachings, History, and Practices. Cambridge University Press, 1990, page 28.
- Bhikkhu Bodhi, In the Buddha's Words. Wisdom Publications, 2005
- Yamaka Sutta, .
- Tyson Anderson, Kalupahana on Nirvana. Philosophy East and West, April 1990, 40(2)
- Alexander Wynne, The Origin of Buddhist Meditation. Routledge 2007
- Dhammapada, verse 92
- 12. Vision of the Universe Abhirati and the Tathagata Aksobhya from Vimalakirti Nirdesa Sutra