Tathāgata (, Devanagari: तथागत) is a Pali and Sanskrit word that the Buddha of the Pali Canon uses when referring to himself. The term is often thought to mean either "one who has thus gone" (tathā-gata) or "one who has thus come" (tathā-āgata). This is interpreted as signifying that the Tathagata is beyond all coming and going – beyond all transitory phenomena. However there are other interpretations and the precise original meaning of the word is not certain.

The Buddha is quoted on numerous occasions in the Pali Canon as referring to himself as the Tathagata instead of using the pronouns me, I or myself. This may be meant to emphasize by implication that the teaching is uttered by one who has transcended the human condition, one beyond the otherwise endless cycle of rebirth and death, i.e. beyond suffering.

Monks, in the world with its devas, Mara and Brahma, in this generation with its ascetics and brahmins, devas and humans, whatever is seen, heard, sensed and cognized, attained, searched into, pondered over by the mind—all that is fully understood by the Tathagata. That is why he is called the Tathagata.

“ ” Anguttara Nikaya 4:23, trans. Nyanaponika Thera and Bhikkhu Bodhi

The term also occurs as a synonym for arahant, identifying one who has attained the ultimate in the holy life. There is even a sense in which such a one is no longer human... "a tathāgata, a superior state of being (uttama-puriso)".

Read more about Tathāgata:  Etymology & Interpretation, The Nature of A Tathagata