WordNet sense 1 (of or relating to or resembling a nebula):

nebular

WordNet sense 2 (lacking definite form or limits):

cloudy, nebulose

WordNet sense 3 (not clearly defined or easy to perceive or understand):

indistinct

WordNet sense 4 (lacking definition or definite content):

unfixed

WordNet sense 5 (vague or not clearly defined or stated):

indefinite