H. P. Blavatsky

Tad‐aikya. “Oneness”; identification or unity with the Absolute. The universal, unknowable Essence (Parahrahm) has no name in the Vedas but is referred to generally as Tad, “ That”.

Source: H. P. Blavatsky, The Theosophical Glossary, art. Tad‐aikya