What tree did Buddha sit under?
The Buddha achieved enlightenment while sitting under a fig
tree,whose species is now called ficus religious. The specific tree
is called The Bodhi tree, where Bodhi means Enlightenment.
The Buddha just before he became a Buddha as a Bodhisattva, sat
under the Assattha (Pali), often called the Bodhi-tree, and
thereupon attained enlightenment becoming a Buddha. There is,
however, more to this. The Pali word Assattha is probably the past
participle of the Pali word, 'assasati' which can literally mean
"enter by the breath". This makes sense if we understand that
Buddhahood is achieved by access to the fourth meditation or dhyana
(Jp., Zen) which utilizes the breath to finally transcend it. Here
the mind reaches its apex. Mind awakens to its own immaculate
nature, hence, Buddha.
That would be the "Bodhi Tree."
Although there are many thoughts on this question the original
tree, under which Buddha received the enlightenment was Ficus
benghalensis, the Banyan.