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 saṅkhāra-kkhandha (s. khandha) and is mostly known by the name upekkhā. In its widest sense it is associated with all pure consciousness (s. 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 factors; 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). (App.).

Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA