The Collaborative International Dictionary of English v.0.48:

Alcahest \Al"ca*hest\, n.
   Same as Alkahest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alcahest
    n 1: hypothetical universal solvent once sought by alchemists
         [syn: alkahest, alcahest, universal solvent]