Unidirectional means "in one direction". It is a transmission in which only one directional transmission channel is available, but no return channel.
The receiver of a message is therefore not able to confirm this to the sender. Since the receivers of a unidirectional transmission cannot check the message to ensure that it is free of errors, the transmission mechanisms are more complex than those of bidirectional transmission systems, which operate in both transmission directions. Typical unidirectional transmissions are transmissions in broadcasting, such as radio and television.
In microphone technology, unidirectional microphones pick up sound primarily from one direction. These are microphones that have a cardioid shape as their directional characteristic. In addition to unidirectional and bidirectional, which transmit in both directions, there is also omnidirectional, which transmits equally in all directions.