The Collaborative International Dictionary of English v.0.48:

Interstellar \In`ter*stel"lar\, a.
   Between or among the stars; as, interstellar space. --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interstellar
    adj 1: between or among stars; "the density of hydrogen in
           interplanetary and interstellar space"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "interstellar":
   Cynthian, anagalactic, asteroidal, astral, astrologic,
   astrologistic, astrologous, astronomic, astrophysical, celestial,
   circumplanetary, cislunar, empyreal, empyrean, equinoctial,
   extragalactic, galactic, heavenly, heliacal, intercosmic,
   interplanetary, intersidereal, lunar, lunary, lunate, lunular,
   lunulate, meteoric, meteoritic, nebular, nebulose, nebulous,
   planetal, planetarian, planetary, planetesimal, semilunar,
   sidereal, solar, sphery, star-spangled, star-studded, starry,
   stellar, stellary, terrestrial, uranic, zodiacal