Bottom type

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The bottom type in type theory is a type that has no values. It is commonly abbreviated as bot, by the up tack symbol (⊥) or by its ASCII approximation _|_. The bottom type is also called the zero or empty type. It contrasts with the top type, which spans all possible values in a system, and a unit type, which has only one value.

The bottom type is frequently used for the following purposes:

To signal that a function or computation diverges; in other words, does not return a result to the caller. (This does not necessarily mean that the program fails to terminate; a subroutine may terminate without returning to its caller, or exit via some other means such as a continuation.)
As an indication of error; this usage primarily occurs in theoretical languages where distinguishing between errors is unimportant. Production programming languages typically use exception handling instead.

In Haskell, the keyword undefined represents a computation for which the result has the bottom type. This means that the computation cannot finish normally for some reason (eg. the program has been asked to find the last entry in an infinite list); an attempt to do so at runtime aborts the program.