What is this part of the jacket called?

[0311Marine]

Cant find the answer anywhere

What do you call that cut on the bottom of the jacket

 

[Dinerman]

vent

 

[flat-top]

Yes^

 

[0311Marine]

ah thank you sir

 

[Rick Blaine]

It can also be called a cloaca.

...if yer' a chicken!