Søren Kierkegaard is widely regarded as the father of existentialism, as his writings explored themes of individual existence, freedom, and the meaning of life.

Jean-Paul Sartre wrote the novel (Nausea), which explores the themes of existentialism through the experiences of its protagonist, Antoine Roquentin.

Albert Camus' novel (The Stranger) explores the theme of the absurdity of existence through the story of Meursault, a man who murders an Arab and is subsequently tried and executed.

Samuel Beckett wrote the play (Waiting for Godot), which explores the themes of existentialism through the experiences of two characters, Vladimir and Estragon, who are waiting for the arrival of a mysterious figure named Godot.

The concept of (angst) in existentialism refers to the anxiety or dread that arises from the awareness of one's own existence and the meaninglessness of life.

Jean-Paul Sartre argued that existence precedes essence, meaning that we are not born with a predetermined nature or essence, but rather we create our own essence through our actions and choices.

The concept of (mauvaise foi) in existentialism refers to the state of bad faith or self-deception, in which individuals deny their own freedom and responsibility and instead adopt conventional or inauthentic ways of being.

Jean-Paul Sartre wrote the book (Being and Nothingness), which is considered one of the most important works of existentialist philosophy.

The concept of (the absurd) in existentialism refers to the conflict between human reason and the irrationality of the world, which leads to a sense of meaninglessness and absurdity.

Søren Kierkegaard argued that (existence is suffering), as humans are constantly confronted with the challenges and difficulties of life.

The concept of (authenticity) in existentialism refers to the state of being true to oneself, living in accordance with one's values, and choosing one's own destiny.

Albert Camus wrote the book (The Myth of Sisyphus), which explores the themes of existentialism through the myth of Sisyphus, who is condemned to roll a boulder up a hill only to have it roll down again.

The concept of (freedom) in existentialism encompasses the ability to choose one's own destiny, the power to create one's own meaning in life, and the responsibility to act in accordance with one's values.

Friedrich Nietzsche famously declared that (God is dead), signifying the decline of traditional religious beliefs and the rise of secularism and individualism.

Existential despair is the feeling of hopelessness and meaninglessness that arises from the awareness of one's own existence and the realization that life is ultimately finite and meaningless.