is the terminal object in the category of types and typed functions. wikipedia 
It should not be confused with the zero or bottom type, which allows no values and is the initial object in this category. Similarly, the Boolean is the type with two values.
In Haskell, Rust, and Elm, the unit type is called () and its only value is also (), reflecting the 0-tuple interpretation. wikipedia
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Empty Containers can be seen as Units.