Solipsism is the theory that self is all that exists.
It’s kind of an oddball worldview first enunciated by the Greek
sophist Gorgias of Leontini around 400 B.C. Gorgias argued that (i) nothing
exists; (ii) even if something exists, nothing can be known about it; and
(iii) even if something could be known about it, knowledge about it can’t
be communicated to others.
Now of course when we refer to someone as “solipsistic”
today, we do not generally mean that they are a philosopher of the Gorgian
school or that they really believe that everything they experience (including
the external world and other people) occurs only in their heads and lacks independent
existence. Most solipsists are not philosophers at all; in fact, they may never
have even heard the word “solipsism”. They have no specific theories of
existence and may never have contemplated reality in the abstract.
They just live and think as if self is all that exists.