A type with 1 value, Unit, though most will see it used via unit. This IS the C or Java-style void type. It usually indicates a "side effect", mutation, or impure code.

-- Data.Unit (Unit, unit)

data Unit = Unit

unit :: Unit
unit = Unit

It's also used to indicate a thunk, a computation that we know how to do but have chosen to delay executing/evaluating until later:

type ComputationThatReturns a = (Unit -> a)

thunk :: forall a. a -> ComputationThatReturns a
thunk a = (\_ -> a)

-- We run the pending computation (force the thunk) by passing
-- unit to it:
runPendingComputation :: ComputationThatReturns a -> a
runPendingComputation thunk = thunk unit