Also also known as "Existential Angst"

"Existential angst", sometimes called dread, anxiety or even anguish, is a term that is common to many existentialist thinkers. It is generally held to be a negative feeling arising from the experience of human freedom and responsibility. The archetypal example is the experience one has when standing on a cliff where one not only fears falling off it, but also dreads the possibility of throwing oneself off. In this experience that "nothing is holding me back", one senses the lack of anything that predetermines one to either throw oneself off or to stand still, and one experiences one's own freedom.

It can also be seen in relation to the previous point how angst is before nothing, and this is what sets it apart from fear that has an object. While in the case of fear, one can take definitive measures to remove the object of fear, in the case of angst, no such "constructive" measures are possible. The use of the word "nothing" in this context relates both to the inherent insecurity about the consequences of one's actions, and to the fact that, in experiencing one's freedom as angst, one also realizes that one will be fully responsible for these consequences; there is no thing in a person (his or her genes, for instance) that acts in her or his stead, and that he or she can "blame" if something goes wrong.

http://en.wikipedia.org/wiki/Existential_angst#Angst