What does an empty seat represent in early Buddhist iconography?

The meditation of the Buddha

Early Buddhist art was largely aniconic, meaning that the Buddha was represented through symbols rather than through a human image. An empty seat, particularly the seat beneath the Bodhi tree, represents the Buddha's presence in meditation and his attainment of enlightenment. The First Sermon is commonly represented by the Dharmachakra, Mahaparinibbana by a stupa, and Mahabhinishkramana by a riderless horse.