Unidirectional
(www.shure.com)
As the name suggests, the unidirectional microphone only picks up a sound source only one direction. The most common type is a cardioid (heart-shaped) response. This has full sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis). This is particularly useful when recording a drum kit for example. As there are usually many other microphones recording the individual parts of a drum kit, each microphone would only want to pick up the drum it is pointing at. Therefore only one third of the ambient noise (surrounding the microphone) is picked up.
A cardioid microphone is most sensitive to sound coming from in front of the microphone ('the bottom the the heart shape') and less sensitive to sound coming from the sides ('off-axis').