[759] Therein, what is equanimity (upekkhā)?
Here a bhikkhu, on the occasion when he develops the path (magga) for rebirth in the form realm (rūpūpapatti): with the abandoning of pleasure and … he enters upon and dwells in the fourth jhāna, accompanied by equanimity — on that occasion there is contact (phassa) … there is non-distraction (avikkhepa): these phenomena (dhammā) are wholesome.
Because that same wholesome kamma of the form sphere (rūpāvacara) has been done, has been accumulated, with the abandoning of pleasure and … he enters upon and dwells in the fourth jhāna — the result (vipāka) — accompanied by equanimity. That which on that occasion is equanimity, the act of being equanimous (upekkhāyanā), the state of being equanimous (upekkhāyitatta), equanimity as deliverance of mind (upekkhā cetovimutti) — this is called equanimity. The remaining phenomena are associated (sampayutta) with equanimity.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.