[754] Therein, what is equanimity (upekkhā)?
Here, on the occasion when a bhikkhu develops the path (magga) for rebirth in the form sphere (rūpūpapatti): with the abandoning of pleasure and … he enters and dwells in the fourth jhāna, which is accompanied by equanimity — whatever on that occasion is equanimity, being equanimous, the state of being equanimous, equanimity as deliverance of mind (cetovimutti): this is called equanimity. The remaining phenomena (dhammā) are associated with equanimity.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.