Solipsism : Thesis to the effect that only I exist (from the Latin: "solus" = "alone" and "ipse" = "self").
There are often distinguished three different main senses of solipsism: Ethical, metaphysical and epistemological. One is solipistic in the ethical sense if one is an egoist, in theory or practice; one is so in the metaphysical sense if one believes there exists nothing but oneself; one is so in the epistemological sense if one holds that all one really knows is one's own experiences and theories.
To use "solipsism" for "egoism" seems mostly confusing, and hence this meaning will not considered here.
Solipsism in the metaphysical sense has been put forward by some philosophers, but usually not in a very serious way. One serious problem the doctrine has, if true, is that it hardly can be stated using ordinary natural language, for the reason mentioned below, when discussing solipsism in the epistemological sense.
Solipsism in the epistemological sense has been put forward by many philosophers, starting with Augustine and Descartes, both of whom considered the argument "cogito ergo sum" (I think, therefore I am - with the suggestion that conversely all I am is what I think) and - Augustine - "fallor ergo sum" (I make mistakes, therefore I am). In either case, the suggestion is that one's self must exist, but the existence of anything else is dubitable.
The reason epistemological solipsism has some importance is that all human knowledge and belief, whatever it is about, starts in the experiences of some human individual, and indeed may never get further than that in cases of mistaken beliefs, fantasies, illusions, impossibilities etc.
The reasons epistemological solipsism is hard to take seriously are mostly of three kinds, of which I first state the fundamental one.
(1) To reason at all one must make assumptions, and indeed make assumptions of things that go beyond one's own present experiences. For if one does not do so, one cannot even test one's guesses. (See: Abduction, Reasoning)
(2) No human being has been raised as a solipsist: Everybody has been raised by other persons who believe that there are other persons with experiences like themselves, whatever else they believe. (See: Other minds, Qualia, Personalism)
(3) Natural language is full of many terms whose standard meanings are not consistent with solipsism, for it is full of ordinary terms that assume the existence of other persons and their experiences, and of things that exist before, after or beyond one's experiences of them. (See: Minimal metaphysics)
Even so, solipsism is philosophically important because it states an extreme and minimalistic thesis - again: only I exist, only my interests, beliefs, experiences exist, there is nothing (that is certain) other than me and myself - that few if any sane persons really believe, and that therefore all avoid by making assumptions that go beyond it, which usually also are not very well-founded.