Natural numbers object (category theory)

A natural numbers object (NNO) is an object in a category with a recursive structure similar to that of the natural numbers. Examples include the set of natural numbers itself, in $𝐒𝐞𝐭$; and the type $\text{T}List\text{app}a$for any inhabited type $a$, in $𝐇𝐚𝐬𝐤$.