What is an Ecological Footprint?
All of the resources which people use for their daily needs and activities come from somewhere, even if not from their immediate surroundings. Food, electricity, and other basic amenities for survival must be produced within the confines of nature, using raw natural resources. Based on this relationship between humanity and the biosphere, an ecological footprint is a measurement of the land area required to sustain a population of any size. Under prevailing technology, it measures the amount of arable land and aquatic resources that must be used to continuously sustain a population, based on its consumption levels at a given point in time. To the fullest extent possible, this measurement incorporates water and energy use, uses of land for infrastructure and different forms of agriculture, forests, and all other forms of energy and material "inputs" that people require in their day-to-day lives. It also accounts for the land area required for waste assimilation.
Source: The Global Development Research Centre http://www.gdrc.org/uem/footprints/index.html