Terminal object

https://en.wikipedia.org/wiki/Initial_and_terminal_objects

The dual notion of an initial object is that of a terminal object (also called terminal element):

T is a terminal object, if for every object X in 𝕮 there exists exactly one morphism X → T.

Terminal objects are also called final objects.

If an object is both initial and terminal, it is called a zero object or null object. A pointed category is one with a zero object.

A is the terminal object:

     B
     ↓
D -> A <- C
     ↑
     D

• every singleton set is a terminal object in the category Set

• there are no zero objects in the category Set