cardiod   

Literally heart-shaped.

Describes any microphone which has this polar pattern.

 
A cardiod microphone will be most sensitive to sounds from the front, less sensitive to sounds from the side and theoretically should not pickup sounds arriving from the rear.  Often used to permit selective amplification in both live and studio applications and in sound reinforcement systems to combat feedback. 

Cardiod microphones are prone to the proximity effect