noun
1.
[ C ] a place where you can buy stamps, send letters, etc. :
Where's the main post office?
You can buy your stamps at the post office.
a post office counter
2.
the ˈPost Office [ sing. ] ( abbr. PO ) the national organization in many countries that is responsible for collecting and delivering letters, etc. :
He works for the Post Office.
3.
[ U ] ( NAmE ) = postman's knock