1. [n]  someone who serves as an intermediary between the living and the dead; "he consulted several mediums"
2. [adj]  of or relating to or connected with spiritualism

SPIRITUALIST is a 12 letter word that starts with S.

1. \Spir"it*u*al*ist\, n.
   1. One who professes a regard for spiritual things only; one
      whose employment is of a spiritual character; an
      ecclesiastic.
   
   2. One who maintains the doctrine of spiritualism.
   
   3. One who believes in direct intercourse with departed
      spirits, through the agency of persons commonly called
      mediums, by means of physical phenomena; one who attempts
      to maintain such intercourse; a spiritist.
   
   

2. \Spir"it*u*al*ist\, a.
   Spiritualistic. --Taylor.