The Collaborative International Dictionary of English v.0.48:

Alfresco \Al*fres"co\, adv. & a. [It. al fresco in or on the
   fresh.]
   In the open-air. --Smollett.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alfresco
    adv 1: outside a building; "in summer we play outside" [syn:
           outside, outdoors, out of doors, alfresco] [ant:
           indoors, inside]
    adj 1: in the open air; "an alfresco lunch"; "an open-air
           theater" [syn: alfresco, open-air(a)]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "alfresco":
   abroad, aerial, aeriform, aery, airish, airlike, airy, atmospheric,
   breezy, en plein air, ethereal, exposed, in the open, light,
   open-air, out of doors, out-of-door, out-of-doors, outdoor,
   outdoors, outside, pneumatic, roomy, tropospheric