With reference to microphones, the opposite of omnidirectional (see WFTD archive, “omnidirectional“). A unidirectional microphone is one which is more sensitive to sound from one direction than from others. The level of “unidirectionality” will vary with the mic’s particular polar pattern (i.e. cardioid, hypercardioid, etc.). There is no such thing as a perfectly unidirectional microphone, but the more unidirectional a mic is, the better it is able to reject off-axis sound, producing more isolated signals.