One of the biggest questions humans have ever asked is: Are we alone in the universe? There's a field of science dedicated to this question known as the Search for Extraterrestrial Intelligence (SETI). In order for SETI scientists to do their job they had to agree on what makes certain life Intelligent, so they'd know what to look for.
SETI scientists came to the consensus that intelligence is associated with the ability to create technology. Some of our best technologies emit radio waves and so SETI has concentrated their work on searching for radio waves in the universe.
But if we're only searching for life like us, we may be overlooking species that are intelligent in other ways. There are many different types of intelligent life on our own planet.
SETI.org: A History of SETI
BBC: Which life form dominates earth?
Scientific American: How Has the Human Brain Evolved?