A microphone polar (pickup) pattern. Characterized by strong sensitivity to audio from the front of the mic, good sensitivity on the sides (at 90 degrees, 6 dB less than the front), and good rejection of sound from the rear, the cardioid pattern can almost be visualized as a “heart-shaped” pattern (hence its name).
The ability to reject sound from the rear makes cardioid patterns very useful in multi-miking situations, and where it is not desirable to capture a large amount of room ambience. Popular in both studio and live use (where rear rejection cuts down on feedback and ambient noise), cardioid mics are used for a very high percentage of microphone applications.
Keep in mind that like all non-omnidirectional mics, cardioid mics will exhibit pronounced proximity effect (see WFTD archives, “Proximity Effect“).