The DICT Development Group

  simplex
      adj 1: allowing communication in only one direction at a time,
             or in telegraphy allowing only one message over a line at
             a time; "simplex system"
      2: having only one part or element; "a simplex word has no
         affixes and is not part of a compound--like `boy' compared
         with `boyish' or `house' compared with `houseboat'"

  simplex
  
     1.  Used to describe a communications channel
     that can only ever carry a signal in one direction, like a
     one-way street.  Television is an example of ({broadcast)
     simplex communication.
  
     Opposite: duplex.
  
     2.  The simplex method.
  
     (2001-07-21)
  

  SIMPLEX. Simple or single; as, charta simplex, is a deed-poll, of single 
  deed. Jacob's L. Dict. h.t.