Definitions of simplex:

• adjective:   having only one part or element

  Example: "A simplex word has no affixes and is not part of a compound--like `boy' compared with `boyish' or `house' compared with `houseboat'"

• adjective:   allowing communication in only one direction at a time, or in telegraphy allowing only one message over a line at a time

  Example: "Simplex system"