Wordnet 3.0
ADVERB (1)
1. 
 within the realm of possibility; 
- Example: "the weather may conceivably change"
The Collaborative International Dictionary of English v.0.48:
Conceivable \Con*ceiv"a*ble\, a. [Cf. F. concevable.]
   Capable of being conceived, imagined, or understood. "Any
   conceivable weight." --Bp. Wilkins.
   [1913 Webster]
         It is not conceivable that it should be indeed that
         very person whose shape and voice it assumed.
                                                  --Atterbury.
   -- Con*ceiv"a*ble*ness, n. -- Con*ceiv"a*bly, adv.
   [1913 Webster]
WordNet (r) 3.0 (2006):
conceivably
    adv 1: within the realm of possibility; "the weather may
           conceivably change"