In physics and energy economics, EROEI (energy returned on energy invested) is the ratio between the amount of energy expended to obtain a resource, compared with the amount of energy obtained from that resource. When the EROEI of a resource becomes 1 or less, that energy source becomes an energy sink and can no longer be used as a primary source of energy.
For example, when oil was originally discovered, it took on average one barrel of oil to find, extract, and process about 100 barrels of oil. That ratio has declined steadily over the last century to about 3 in the US (and about 10 in Saudi Arabia) - ultimately this ratio will fall under one, necessitating the modern world to switch to other forms of energy to power civilization.
See also