Early travellers to India had observed that Indian traders and merchants frequented the tree for the shade. Indian merchants are called Banias in local languages. The Portuguese picked up the word and passed it on to the English. Eventually, the name Banyan became the name of the tree.
The tree's scientific name is Ficus Benghalensis. It's a Fig. Its seeds germinate in the cracks and crevices on a host tree, although cracks on buildings and structures are fine as well. The seeds germinate and send down roots to the ground. The host tree is enveloped and starved for light. This is why these trees are also known as strangler fig trees.
Older banyan trees are characterized by their aerial prop roots which grow into thick woody trunks. With age, these become indistinguishable from the main trunk. These trees can spread out laterally using these prop roots to cover a wide area. left alone these trees can survive eternally.