What is "Cardioid Microphone"?

Cardioid Microphone

A cardioid microphone is characterized as a unidirectional microphone. The polar pattern of the cardioid exhibits full sensitivity on-axis, whereas the sensitivity at 180° in principle is -∞; in practice >20 dB.

The -3 dB angle is at ±65.5°. The -6 dB angle is at ±90°.

In principle, this characteristic can be obtained by combining equal parts of the output of an omnidirectional microphone and a bidirectional microphone.
The polar equation is: 0.5 (1+ cosϕ)