Burnham, Douglas, and George Papandreopoulos. "Existentialism." (INTERNET ENCYCLOPEDIA OF PHILOSOPHY).

Existentialism is a catch-all term for those philosophers who consider the nature of the human condition as a key philosophical problem and who share the view that this problem is best addressed through ontology. This very broad definition will be clarified by discussing seven key themes that existentialist thinkers address. Those philosophers considered existentialists are mostly from the continent of Europe, and date from the 19^th and 20^th centuries. Outside philosophy, the existentialist movement is probably the most well-known philosophical movement, and at least two of its members are among the most famous philosophical personalities and widely read philosophical authors. It has certainly had considerable influence outside philosophy, for example on psychological theory and on the arts. Within philosophy, though, it is safe to say that this loose movement considered as a whole has not had a great impact, although individuals or ideas counted within it remain important. Moreover, most of the philosophers conventionally grouped under this heading either never used, or actively disavowed, the term ‘existentialist’. Even Sartre himself once said: “Existentialism? I don’t know what that is.” So, there is a case to be made that the term – insofar as it leads us to ignore what is distinctive about philosophical positions and to conflate together significantly different ideas – does more harm than good.

In this article, however, it is assumed that something sensible can be said about existentialism as a loosely defined movement. The article has three sections. First, we outline a set of themes that define, albeit very broadly, existentialist concerns. This is done with reference to the historical context of existentialism, which will help us to understand why certain philosophical problems and methods were considered so important. Second, we discuss individually six philosophers who are arguably its central figures, stressing in these discussions the ways in which these philosophers approached existentialist themes in distinctive ways. These figures, and many of the others we mention, have full length articles of their own within the Encyclopedia. Finally, we look very briefly at the influence of existentialism, especially outside philosophy. . . .

http://www.iep.utm.edu/existent/