ex-is-ten-tial-ism (ek"sis-ten'shel-iz-em) n. (1) A philosophy that emphasizes the uniqueness and isolation of the individual experience in a hostile or indifferent universe, regards human existence as unexplainable, and stresses freedom of choice and responsibility for the consequences of one's acts. (2) a humanistic philosophy stating that each man is responsible for forming his self and must with his free will oppose his uncertain, purposeless, and seemingly hostile environment.