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, 6dB 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).