Post - The small "finger" that sticks out from the back of the buckle that goes into a punched hole in the belt, resulting in a belt size that is appropriate for you. Also sometimes referenced as a "flopper", "finger" or "stay".
The holes on a belt are known as notches. When putting your belt around your waist, you secure the posts (the small fingers that go through the 'holes) inside the notches and you can make your belt tighter or looser depending on which notch you choose. There are generally five notches on a belt.
A belt wedgie is when someone slips a belt between the holes of your underwear and pulls up on the belt. If you want to give one to yourself, you can put the belt through the holes of your underwear and either pull up of fasten the belt to something and hang.