The Collaborative International Dictionary of English v.0.48:

Aeriform \A"["e]r*i*form\ (?; 277), a. [L. a["e]r air + -form:
   cf. F. a['e]riforme.]
   Having the form or nature of air, or of an elastic fluid;
   gaseous. Hence fig.: Unreal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

aeriform
    adj 1: resembling air or having the form of air [syn:
           aeriform, airlike]
    2: characterized by lightness and insubstantiality; as
       impalpable or intangible as air; "figures light and aeriform
       come unlooked for and melt away"- Thomas Carlyle; "aerial
       fancies"; "an airy apparition"; "physical rather than
       ethereal forms" [syn: aeriform, aerial, airy, aery,
       ethereal]