noun
eldest grandson of an emperor in the line of succession
A formal imperial title; used in historical and modern contexts for the eldest grandson of the reigning emperor who is heir apparent.
皇太孫は天皇の長男の長男にあたる。
The Kōtaison is the eldest son of the Emperor's eldest son.
皇太子 is the crown prince (emperor's son), while 皇太孫 is the emperor's grandson in the line of succession.
Composed of 皇 (imperial), 太 (great), and 孫 (grandchild). The term follows the pattern of 皇太子 (crown prince) but for the grandson.