[Peguy]

That's the common definition of Existentialism.

Overall it's a philosophy concerning the nature of human existence as opposed to abstract concepts like traditional philosophy. Now what the hell human existence means is anybody's guess.

The standard division is between "Atheist" Existentialists like Sartre, Heideggar, Jaspers, etc.

Then you have "Theistic" Existentialists like Kierkegaard, Berdyaev, Marcel, Tillich, Shestov, Buber, etc.

Naturally as a Catholic I tend towards the latter.