Upekkha is Pali for "equanimity." It is a mind in balance, free of discrimination, above distinction, and rooted in insight. This balance is not indifference, but active mindfulness. Because it is rooted in insight of anatman, it is not unbalanced by the passions of attraction and aversion and is free of the extremes of joy or sorrow.
Together with metta (loving kindness), karuna (compassion) and mudita (sympathetic joy), upekkha is one of the Four Divine States or Four Immeasurables.