Hikikomori are both an increasingly prominent archetype and an actual social phenomenon in Japan. The term refers to a certain type of person who becomes reclusive and withdraws from society at large.
A hikikomori is someone who secludes themselves and wants to remain isolated. They start to lose their social skills/graces and don't follow the norms of the outside world. It is difficult for them to interact with people outside of their own space they've confined themselves to.
The most famous example of a hikikomori is in the franchise Welcome to the NHK where the viewer follows the progress of a recovering hikikomori named Tatsuhiro Satou.