In 1964, Russian-Soviet astrophysicist and radio astronomer Nikolai Kardashev wrote a paper where he described how alien civilizations could be categorized based on the energy they could harness. At the time, Kardashev was trying to suggest the kinds of transmissions astronomers should be looking for in their pursuit of finding extraterrestrial intelligence (ETI). However, this paper spawned a means of classifying ETIs based on the amount of energy they could harness.
This has come to be known as the Kardashev Scale, which categories civilizations into the planetary, stellar, and galactic scales. Today, it remains a central feature in the Search for Extraterrestrial Intelligence (SETI).