tathatta : [nt.] the state of being so; the truth.

A.P. Buddhadatta Mahathera, Concise Pali-English and English-Pali Dictionary [available as digital version from Metta Net, Sri Lanka]

Tathatta (nt.) [*tathātvaŋ] "the state of being so," the truth, Nibbāna; only in foll. phrases: (a) tathattāya paṭipajjati to be on the road to (i. e. attain) Nibbāna D i.175, similarly S ii.199; S ii.209 (paṭipajjitabba being conducive to N.); Miln 255; Vism 214. -- (b) tathattāya upaneti (of a cittaŋ bhāvitaŋ) id. S iv.294=M i. 301; S v.90, 213 sq. -- (c) tathattāya cittaŋ upasaŋharati id. M i.468. -- abl. tathattā in truth, really Sn 520 sq. (cp. M Vastu iii.397).

Pali-English Dictionary, TW Rhys Davids, William Stede,