Street level definition of Existentialism

Existentialism (I'm referring to atheistic existentialism which pervaded popular culture throughout the 20th century) is said to be notoriously hard to define. I think that is probably because most people don't have a concrete understanding of it, if there is one. In reading James Sire's The Universe Next Door I came across a pretty good explanation of it. From memory here it is.

Existentialism is not really a philosophy but a reaction to a philosophy (or philosophical worldview); in this case a reaction to nihilism.

I.e. an existentialist is first a nihilist. He considers the world and existence to be ultimately meaningless and absurd. Then - from that foundation - he (as an existentialist) proceeds to determine to create himself and his world in a kind of heroic action that is still meaningless yet at least is macho (or pleasurable, or self-sacrificial, or successful in existentialist terms which is becoming what is called a 'strong poet').

A strong poet, in existentialist terms, is a person who muscles his view and language of something into the world to where other people adopt it as part of reality. In so many words. This could be in music, architecture, psychology, literature, whatever.

There seems to be a low hum of hedonism as well in the existentialist quest, or manifesting essence. Get your cigarettes, your women, your adrenaline rushes while you can type of hedonism. Not necessarily really decadent hedonism (because existentialists seem to operate by a code that would look down on such things; which gets at the subject of how close really existentialism is to a practical sort of Christianity)...

As for the existentialist saying: existence precedes essence... That means, in existentialist terms, you are nothing until you make of yourself something. Even though it's all meaningless anyway...