The Collaborative International Dictionary of English v.0.48:

existentialism \ex`is*ten"tial*ism\, n. (Philosophy)
   a philosophical theory or attitude having various
   interpretations, generally emphasising the existence of the
   individual as a unique agent with free will and
   responsibility for his or her own acts, though living in a
   universe devoid of any certain knowledge of right and wrong;
   from one's plight as a free agent with uncertain guidelines
   may arise feelings of anguish. Existentialism is concerned
   more with concrete existence rather than abstract theories of
   essences; is contrasted with rationalism and empiricism;
   and is associated with Kierkegaard, Heidegger and Sartre, as
   well as others.
   [PJC]

WordNet (r) 3.0 (2006):

existentialism
    n 1: (philosophy) a 20th-century philosophical movement chiefly
         in Europe; assumes that people are entirely free and thus
         responsible for what they make of themselves [syn:
         existentialism, existential philosophy, existentialist
         philosophy]