DuckCorp Dico

Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   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'"