The Great Filter is a hypothesis in astrobiology proposing that something dramatically limits the prevalence of observable Alien Life in the universe. It suggests a "filter" or barrier that prevents most life, including advanced Civilization, from reaching a stage where it could be widely detected, thus offering a possible solution to the Fermi Paradox. This barrier could be in our past, meaning we have already passed it, or it could lie ahead, posing a future existential challenge.