Tatra-majjhattatā: 'equanimity, equipoise, mental balance' lit., 'remaining here and there in the middle', is the name for a high ethical quality belonging to the sankhāra-khandha see: khandha and is mostly known by the name upekkhā In its widest sense it is associated with all pure consciousness see: Tab. II.. tatra-majjhattatā is called the 'keeping in the middle of all things'. It has as charactcristic that it effects the balance of consciousness and mental properties; as nature function; rasa that it prevents excessiveness and deficiency, or that it puts an end to partiality; as manifestation, that it keeps the proper middle; Vis.M XIV.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.