The Collaborative International Dictionary of English v.0.48:

Immaterial \Im`ma*te"ri*al\ ([i^]m`m[.a]*t[=e]"r[i^]*al), a.
   [Pref. im- not + material: cf. F. immat['e]riel.]
   1. Not consisting of matter; incorporeal; spiritual;
      disembodied.
      [1913 Webster]

            Angels are spirits immaterial and intellectual.
                                                  --Hooker.
      [1913 Webster]

   2. Of no substantial consequence; without weight or
      significance; unimportant; as, it is wholly immaterial
      whether he does so or not.

   Syn: Unimportant; inconsequential; insignificant;
        inconsiderable; trifling.
        [1913 Webster]

WordNet (r) 3.0 (2006):

immaterial
    adj 1: of no importance or relevance especially to a law case;
           "an objection that is immaterial after the fact" [ant:
           material]
    2: without material form or substance; "an incorporeal spirit"
       [syn: incorporeal, immaterial] [ant: corporeal,
       material]
    3: not consisting of matter; "immaterial apparitions"; "ghosts
       and other immaterial entities" [syn: immaterial,
       nonmaterial] [ant: material]
    4: not pertinent to the matter under consideration; "an issue
       extraneous to the debate"; "the price was immaterial";
       "mentioned several impertinent facts before finally coming to
       the point" [syn: extraneous, immaterial, impertinent,
       orthogonal]
    5: (often followed by `to') lacking importance; not mattering
       one way or the other; "whether you choose to do it or not is
       a matter that is quite immaterial (or indifferent)"; "what
       others think is altogether indifferent to him" [syn:
       immaterial, indifferent]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "immaterial":
   adrift, aerial, airy, apparitional, asomatous, astral, back-burner,
   beside the mark, beside the point, beside the question, bodiless,
   celestial, decarnate, decarnated, dinky, discarnate, disembodied,
   dispensable, ephemeral, ethereal, evanescent, extramundane,
   extraneous, extrinsic, fanciful, flimsy, foreign, ghostly,
   heavenly, illusory, imaginary, impalpable, impertinent,
   imponderable, inadmissible, inapplicable, inapposite,
   inappreciable, inappropriate, incidental, inconsequent,
   inconsequential, inconsiderable, incorporate, incorporeal,
   inessential, inferior, insensible, insignificant, insubstantial,
   intangible, irrelative, irrelevant, light, little, merely nominal,
   metaphysical, minor, minute, negligible, nihil ad rem,
   nonessential, nonmaterial, nonphysical, nonsubstantial,
   not at issue, not real, not vital, nugatory, occult,
   off the subject, otherworldly, out-of-the-way, parenthetical,
   petit, petty, phantom, psychic, shadowy, slight, small, spiritual,
   subjective, supernatural, technical, transmundane, trifling,
   trivial, unactual, unconcrete, unearthly, unembodied, unessential,
   unextended, unfleshly, unimportant, unimpressive, unnoteworthy,
   unphysical, unreal, unrealistic, unsolid, unsubstanced,
   unsubstantial, unworldly, weightless



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IMMATERIAL. What is not essential; unimportant what is not requisite; what
is informal; as, an immaterial averment, an immaterial issue.
     2. When a witness deposes to something immaterial, which is false,
although he is guilty of perjury in foro conscientiae, he cannot be punished
for perjury. 2 Russ. on Cr. 521; 1 Hawk. b. 1, c. 69, s. 8; Bac. Ab.
Perjury, A.