Kokuyaku Constitution defined laws that were formulated by representatives of the people and approved by the ruler (in Japan's case it would be the Emperor) and this procedure served as a sort of promise between the ruler and his people.
国約憲法（こくやくけんぽう）とは、国民代表の会議によって憲法を作成して君主（日本の場合は天皇）に承認させる手続を経る事で国民と君主間の約束事として位置づけられた憲法のこと。