I looked it up on Wiki and the entry Existentialism said that that a definition has never really been agreed upon. Emerges before the French thinkers of the 20th century, like Albert Camus and Jean-Paul Sartre. I believe the precursor of the movement was the German's Martin Heidegger. Sometimes, I can label myself as nihilist-existentialist sorta person. Existentialism deals with the "anxiousness" of one person in this world. Being lost and not giving a fuck about the world. In other words, feeling so alienated in space from everything and everything does not necessarily hold a meaning. I feel like I am floating in the air with my own inner thoughts whether in a crowd or alone.
Existentialism basically boils down to this: As humans, we have all come to have our realities framed around that identity of being human, and everything that entails, from a personal perspective. Regardless of what external verification and objective logic states, the world is just a reflection of our human identity. Nothing matters beyond the personal human experience.
Nihilism, which was developed with another goal in mind, though often included as a part of existentialism, states that nothing matters, not even that human identity by which you judge the world, because even it is a fabricated lie.
If you are a monist Hindu, apatheist / impersonalist, or a realist / physicalist ... sometimes your attitudes may resemble that of the existentialist or nihilist. I wonder if it is something like anarchism that was busier trashing the established order than building one of its own?