Alternatively referred to as SPP (Standard Parallel Port), unidirectional refers to data either sent or received in one direction. For example, the parallel port on a computer can be set to unidirectional, allowing the computer to send data to a printer but not allow that printer to communicate back to the computer.
Unidirectional ports were found on computers manufactured before 1995, capable of transmitting data 4-bits at a time, at 40 to 60 KBps over a six foot cable. This standard was later moved into the IEEE-1284, updated, and now known as Centronics mode.
Unidirectional may also describe a type of microphone that is only capable of picking up sound in one direction.