The most precise definition of habitat is that it is the area that provides all of the resources necessary for a population of a given species to be self sustaining. For animals, this often means that "habitat" covers a lot of ground (no pun intended). The habitat of the whooping crane includes the karst swamps of the Northwest Territories (nesting habitat), coastal marshes of Texas (wintering habitat), and numerous wetland complexes (migratory habitat) in between.
Since plants don't move (very quickly), their habitat is generally easier to quantify, a 10 acre oak woodland may well be all the habitat a population of yellow lady's slipper orchid needs to sustain itself, for example. Even this definition is a bit fuzzy, though, we often don't know enough about plants to say with great certainty what their habitat requirements are.
A more general definition of habitat is that it is the area where an organism naturally lives. The problem with that definition is that "natural" can be interpreted a variety of ways. It is often assumed to mean "without human influence". Thing is, no plant on Earth today is free from human influence, so you end up having a debate about what "natural" means. On the other hand, if you remove "natural", you end up with a definition that is essentially meaningless.
