In astrology a decumbiture chart is an inception chart that is cast for the moment that a person lays down or takes to their bed as a result of becoming ill.
The decumbiture chart is essentially an inception or commencement chart for the illness itself, and it is used in medical astrology in order to determine information about the nature and course of the illness.
The practice is apparently very old, dating back to the early Hellenistic tradition. It was especially used in late Medieval and Renaissance Europe when most doctors had some training in astrology, in order to help diagnose and treat patients.
The term is derived from the Latin word decumbo, which means “to lie down” or “to fall down”. The use of this term by the Latin authors is probably in imitation of the use of the word kataklisis (κατάκλισις) to refer to the same concept in Greek astrological texts. Kataklisis also means “to lie down” or “take to one’s bed”.