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'"

The Free On-line Dictionary of Computing (30 December 2018):

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)


Bouvier's Law Dictionary, Revised 6th Ed (1856):

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