Definitions by the International Union for the Conservation of Nature (IUCN) (IUCN 2001):
Extent of occurrence is defined as the area contained within the shortest continuous imaginary boundary that can be drawn to encompass all the known, inferred or projected sites of present occurrence of a taxon, excluding cases of vagrancy. This measure may exclude discontinuities or disjunctions within the overall distribution of a taxon (e.g., large areas of obviously unsuitable habitat) (but see area of occupancy below). When the range extent is estimated for an Element that has large areas of obviously unsuitable or unoccupied habitat that should be excluded from the calculation, the range extent will be overestimated by using a minimum convex polygon (also called a convex hull) to calculate the area. In these cases, the α-hull is recommended. The α-hull can be estimated by making a Delauney triangulation of the points in a sample (connect the points with lines, constrained so that no lines intersect), and then deleting lines that are longer than two times the average line length. The range extent is then the sum of enclosed areas.
Area of occupancy is defined as the area within its extent of occurrence (see definition above), which is occupied by a taxon, excluding cases of vagrancy. The measure reflects the fact that a taxon will not usually occur throughout the area of its extent of occurrence, which may contain unsuitable or unoccupied habitats. In some cases (e.g. colonial nesting sites, feeding sites for migratory taxa) the area of occupancy is the smallest area essential at any stage to the survival of existing populations of a taxon. The size of the area of occupancy will be a function of the scale at which it is measured, and should be at a scale appropriate to relevant biological aspects of the taxon, the nature of threats and the available data. Unless the area of occupancy is estimated through high intensity sampling, the estimated area of occupancy should generally be calculated by conceptually using a standard grid cell size of 2 km (a cell area of 4 km2) in order to ensure consistency and comparability of results.
Figure 1 illustrates the differences between range extent and area of occupancy.
Citation for Burgman and Fox (2001)