Koreisai is a collective term for shunkikoreisai (imperial ceremony of ancestor worship formerly held on the vernal equinox) and shukikoreisai (imperial ceremony of ancestor worship formerly held on the autumnal equinox).
皇霊祭（こうれいさい）は、すなわち春季皇霊祭・秋季皇霊祭をさす。