The Great Filter is a very improbable step leading to an interstellar civilization, its existence is a proposed explanation for the Fermi Paradox. Robin Hanson coined the term in his 1998 essay The Great Filter - Are We Almost Past It?.
Given our actual knowledge of the universe, it is uncontroversial to assume that many other habitable planets - fulfilling the conditions for life to flourish until it develops into a interstellar civilization - are bound to exist. At the same time, there is a complete lack of evidence for the existence of other civilizations. Those two facts implies the existence of a extremely improbable step, a Great Filter, in the path leading from the basics conditions for originating life to the emergence of interstellar civilizations.
The Great Filter might be the transition from prokaryotes to eukaryotes, or from unicellular to multicellular life forms, or yet the appearance of photosynthesis. Should this be the case, we have already passed through the improbable region, the filter is behind us and we shouldn’t worry. But the hard step might also be ahead of us: to survive the creation of nuclear bombs, AGI biotechnology, nanotechnology or an asteroid impact 1. In this scenario, we are passing or will pass through the improbable region and we should worry. Estimating the location of the Great Filter is a crucial task for dealing with Existential risk. Many efforts have been made in that direction, but much remains uncertain....