What is a value called, that signals that nothing has happened?

Question

I'm at a loss for what to call that or even what to search for.

What I'm looking for is something like this:

value = None

if something_is_true:
    value = 123

if value is not None:
    send(value)

What is the use of None called in this instance? In my head it sounds something like Cardinal Value, although that is not it.

Answer 1

The word you're probably looking for is "sentinel", that is a variable that triggers some behavior depending on whether it's set or not.

For further information see Sentinel value on Wikipedia.