The Collaborative International Dictionary of English v.0.48:

Alkahest \Al"ka*hest\, n. [LL. alchahest, F. alcahest, a word
   that has an Arabic appearance, but was probably arbitrarily
   formed by Paracelsus.]
   The fabled "universal solvent" of the alchemists; a menstruum
   capable of dissolving all bodies. -- Al`ka*hes"tic, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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