deceased grandmother of the emperor
Highly formal, historical term referring specifically to the late grandmother of a reigning emperor. Used in official records or ceremonial contexts.
A ceremony was held to report to the spirit of the deceased imperial grandmother.
Compound of 皇 (imperial) + 祖 (ancestor) + 妣 (deceased mother/grandmother). The exact historical derivation is uncertain; the term is a formal Sino-Japanese construction used in imperial household contexts.