{i} one who supports existentialism (20th-century philosophical movement stressing the subjective aspect of human existence and his responsibility for it)
a philosopher who emphasizes freedom of choice and personal responsibility but who regards human existence in a hostile universe as unexplainable relating to or involving existentialism; "existentialist movement"; "existentialist philosophy"; "the existentialist character of his ideas
If you describe a person or their philosophy as existentialist, you mean that their beliefs are based on existentialism. existentialist theories