From Rigpa Wiki
The Vajra Seat (Skt. vajrāsana; Tib. རྡོ་རྗེ་གདན་, dorjeden; Wyl. rdo rje gdan) is the very place where the Buddha reached enlightenment, under the bodhi tree, by the river Nairanjana in today's Bodhgaya. In Buddhist cosmology it is considered to be the centre of the universe and the place where the 1002 buddhas of this Fortunate Aeon reach enlightenment.