When watching this speech by the Atari founder, he says (at 09:19):
If you're a true existentialist [...] you want to have an interesting life.
Thus, if someone says he or she is an existentialist, in the way he did, what might this mean in essence? I looked at the Wikipedia article for Existentialism; yet I'm still unsure as to what it means in a nutshell if stated in a colloquial way.
Jostein Gaarder gave the explanation which I have found most satisfying in his book, Sophie’s World. I have pieced together several excerpts below.
Existentialism is a collective term for several philosophical currents that take man’s existential situation as their point of departure. Sartre said that “existentialism is humanism”. Existentialists start from nothing but humanity itself. Sartre’s allegiance was to what we might call an atheistic existentialism. His philosophy can be seen as a merciless analysis of the human situation when “God is dead”. The being of man is not the same as the being of things. Man is conscious of his own existence. Man’s nature is not fixed in advance. It is therefore useless to search for the meaning of life in general. We are condemned to improvise. When people realize they are alive and will one day die—and there is no meaning to cling to—they experience dread. Man feels alien in a world without meaning, and this creates a sense of despair, boredom, nausea, and absurdity. We haven't asked to be created as free individuals. Nevertheless we are free individuals, and this freedom condemns us to make choices throughout our lives. There are no eternal values or norms we can adhere to, which makes our choices even more significant. We are totally responsible for everything we do. It is we ourselves who create meaning in our own lives. To exist is to create your own life.
I think the most concise (and yet still useful) explanation you are likely to find is Sartre's brief essay Existentialism is a Humanism.
If you read that and still have questions, post them.
I believe Heidegger set the foundations of existentialism in his attempt to renew the occidental metaphysic by analyzing the existence of the man, he calls dasein by opposition to the Being in his seminal work “Being end time”. The dasein has an unformulated worry, he is cast in the world and abandoned by the Being, he exists by opposition to the Being which is. In German and French there are two words for being, the fact to be and the Being. The ontology of the dasein, the analysis of its existence is certainly the foundation of existentialism even if Heidegger did not recognized himself as an existentialist. The dereliction of the man is at the root of this philosophical movement.
According to itself, Existentialism has not been invented by someone. It is as old as Man itself. The definition of it is a set of principals that can all be derived systematically from the following ideas without a specific order ( they are all equivalent propositions):
1) I am the centre of the world. The only absolute and certain truth is that I exist. And the word world is nothing but the one that I perceive as such because I am the one who invented this word, it didn't invented itself nor do I found it under a tree.
2) I have to choose what is best for me alone.
3) I know I will die therefore I am free to choose. Intellectual manipulation is therefore impossible but people choose to be manipulated because they want to escape themselves as the centre of their world.
4) Those followers are called the crowd. They follow so that they are not responsible of their individual existence and do not suffer from the anxiety of the "I exist". Namely, that any life leads to death and that existence is an eternal trip onto the unknown.
...so on and so forth....
Given that you have already perused Wiki, have a look at the Stanford Encyclopedia of Philosophy entry here: https://plato.stanford.edu/entries/existentialism/. Colloquially, I shall simply quote myself here: Does the word "absurd" have specialized meaning in philosophical discourse?:
"…existentialists argue that human beings cannot escape asking the question, “What is the meaning of existence?” They deny, however, that there is an answer to this question, and reject every scientific, teleological, metaphysical, or human-created end that purport to provide an adequate answer.
For instance, Sartre posits/presupposes that “existence precedes essence.” Thus while the meaning of life question seeks an a priori metaphysically universal, “why are we here”, “meaning of it all,” answer “from the beyond,” so to speak, no such answer can exist because there is nothing [knowably] there that can provide it (given the presupposition). Or see Camus’ take (from https://plato.stanford.edu/entries/camus/):
Thus, while accepting that human beings inevitably seek to understand life’s purpose, Camus takes the skeptical position that the natural world, the universe, and the human enterprise remain silent about any such purpose. Since existence itself has no meaning, we must learn to bear an irresolvable emptiness. This paradoxical situation, then, between our impulse to ask ultimate questions and the impossibility of achieving any adequate answer, is what Camus calls the absurd. Camus’s philosophy of the absurd explores the consequences arising from this basic paradox
Camus’s understanding of absurdity is best captured in an image, not an argument: of Sisyphus straining to push his rock up the mountain, watching it roll down, then descending after the rock to begin all over, in an endless cycle. Like Sisyphus, humans cannot help but continue to ask after the meaning of life, only to see our answers tumble back down."
The question "what is an existentialist" is as broad as asking what is an analytic philosopher. The term existentialism (like the term "analytic philosophy") is better conceived as a group of methods, rather than a school of thought, whose focus is the domain of issues that have historically arisen when one finds oneself thrust-into/in existence, and wonders WTF, "why am I here(?)," in the broadest sense.
The question takes as its starting point the experience of the human subject who so finds him/herself— not only the thinking/rational subject, but the acting, feeling, living human individual (i.e. I'm "here," so what now? why so? and how should/do I feel about it,and why so, etc). For the answers of system building existentialist philosophers see Sartre or Heidegger; For Christian existentialists see (especially) Kierkegaard or Jaspers; for a nihilistic existentialist see Nietzsche, for a literary existentialist see Camus, for a sociological existentialist see Ortega-Gasset, etc.
