Gekokujō refers to the concept of 'lower overcoming the upper' in Japanese history, particularly during the Sengoku period, when it was common for lower-ranking samurai or peasants to challenge and defeat their social superiors. This phenomenon was a reflection of the social upheaval and decentralization of power that characterized the Sengoku era, where traditional hierarchies were disrupted, leading to new power dynamics among daimyō and samurai.