WordNet sense 1 (anyone who does not belong in the environment in which they are found):

alien, stranger

WordNet sense 2 (an unknown and unexplored region):

terra incognita, unknown region

WordNet sense 3 (a variable whose values are solutions of an equation):

unknown quantity

WordNet sense 4 (not suspected or believed likely):

unsuspected

WordNet sense 5 (not yet identified):

unidentified

WordNet sense 6 (not yet discovered):

undiscovered

WordNet sense 7 ((usually used with `to') occurring or existing without the knowledge of):

unbeknown

WordNet sense 8 ((of unknown regions) not yet surveyed or investigated):

uncharted

WordNet sense 9 (not famous or acclaimed):

obscure, unsung

WordNet sense 10 (not bringing honor and glory):

dishonorable, inglorious

WordNet sense 11 (not known before):

strange

WordNet sense 12 (not known or well known):

unfamiliar

WordNet sense 13 (being or having an unknown or unnamed source):

nameless, unnamed

WordNet sense 14 (having no known name or identity or known source):

anonymous, anon.