The return type of functions that don’t explicitly specify a return type, that is, an empty tuple ().

Declaration

typealias Void = ()

Discussion

When declaring a function or method, you don’t need to specify a return type if no value will be returned. However, the type of a function, method, or closure always includes a return type, which is Void if otherwise unspecified.
Use Void or an empty tuple as the return type when declaring a closure, function, or method that doesn’t return a value.

// No return type declared:
func logMessage(_ s: String) {
    print("Message: \(s)")
}

let logger: (String) -> Void = logMessage
logger("This is a void function")
// Prints "Message: This is a void function"

Reference: Void