Indexical uncertainty is irreducible subjective uncertainty induced by anthropically expecting to be in more than one possible future world.
Suppose you are an AI and shortly 2 instances of you will see “Hello green world” and 3 instances will see “Hello red world”. If you are a conscious AI, then you should anticipate seeing a green world with 40% probability, and a red world with 60% probability, and this subjective uncertainty is irreducible because it reflects an objective state of affairs where different versions of you see different things. To obtain full knowledge of this state of affairs obtaining later is to be left with subjective uncertainty now about “what I will see happen next”.
On the Many-Worlds Interpretation, quantum uncertainty is irreducible because it is indexical uncertainty; the amplitudes in configuration space diverge into blobs of different measure, and we find ourselves in all blobs, somehow with a weight proportional to the measure of amplitude inside our blob.
I think the phrase “more than one possible future world” is misleading and should be changed. The paradigmatic cases of indexical uncertainty (including both given examples of the five-AI-clones case and of Everettian quantum mechanics) involve no certainty about which objective possible world is actual, and only involves self-locating uncertainty about my location within a possible world.
So it would be more accurate to say that it is irreducible subjective uncertainty over multiple centered possible worlds or multiple locations with a given possible world.