A Bonsai is simply a plant that has been given the features of a fully grown mature tree but its size is considerably reduced as compared to the original tree, if it would have been grown on the ground.
Bonsai is not a plant species, rather its a technique, a technique of dwarfing plants in a pot to get a miniature tree like appearance.
Almost all tree species can be transformed into a Bonsai. Apart from its tree like look, there is one more factor which makes bonsai, a Bonsai, & that is Perspective. Without that vision, which is required to understand a Bonsai tree, it will only be called a plant.