Anguish is a term used in philosophy, often as a translation from the Latin for angst. It is a paramount feature of existentialist philosophy, in which anguish is often understood as the experience of an utterly free being in a world with zero absolutes (existential despair). In the theology of Kierkegaard, it refers to a being with total free will who is in a constant state of spiritual fear that his freedom will lead him to fall short of the standards that God has laid out for him.
In the teachings of Sartre, anguish is seen when an utterly captured being realizes the unpredictability of his or her action. For an example, when walking along a cliff, you would feel anguish to know that you have the freedom to throw yourself down to your imminent death.