Flora is a word used to describe the assemblage of plant species that occurs in some particular area or large region. Flora can refer to a modern assemblage of plant species, or to a prehistoric group of species that is inferred from the fossil record. The zoological analogue is known as a fauna, although this word is usually used in reference to a large region. More locally, "vegetation" refers to the occurrence of groupings of plants, often called communities, in some area or region.
The word flora is also sometimes used to refer to a book that describes a taxonomic treatment of plants in some region. Floras of this sort often contain identification keys, diagrammatic, and written descriptions of the species, range maps, and descriptions of habitat.
Plant biogeographers have divided Earth and its regions into floristic units on the basis of their distinctive assemblages of plant species. The species of these large regions (sometimes called biomes, especially in the ecological context) are segregated on the basis of two complexes of factors: (1) geographic variations of environmental conditions, especially climate and to a lesser degree, soil, and (2) physical and ecological barriers to migration, which prevent the distinctive species of floras from mixing together.
In cases where regions have been physically separated for very long periods of time, their differences in plant species are especially great. In particular, isolated oceanic islands often have unique floras, composed of many endemic species of plants that occur nowhere else. For example, islands of the Hawaiian archipelago have been isolated from the nearest mainland for millions of years, and are estimated to have had an original flora of about 2,000 species of angiosperm plants, of which 94-98% were endemic. Unfortunately, many of these unique species have become extinct since these islands were discovered and colonized by humans, first by Polynesians and, more recently and with much greater ecological damages, by Europeans.
In cases where the physical isolation is less ancient, there can be a substantial overlap of genera and species in the floras of different regions. For example, eastern Siberia and the Alaska-Yukon region of North America were physically connected by a land bridge during the most recent era of glaciation, which abated about 14,000 years ago. Reciprocal movements of plants (and some animals, including humans) occurred across that land bridge, and this is indicated today by numerous examples of the occurrence of the same plant species in both regions. For this reason, these regions are considered to have a floristic affinity with each other.