1. [adj]  (telecommunication) 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. [adj]  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'"

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.