The threads about Imperial population got me thinking about what the Imperium counts as a world.
So we all know that 'One Million Worlds' like 40 days and 40 nights is more poetic licence than a hard and fast number. And most fans would agree that 'one million worlds' would not include an airless moon with a missile base or an asteroid with a mine on it.
Worlds have Imperial governors who have complete power as long as they meet certain obligations - cull mutants and psykers, pay tithes/taxes, defend the world, obey other Imperial laws and requests as needed. And certainly their domains may include some other planets/moons/asteroids/stations. Maybe even other star systems if there's important resources and no viable world to support its own governor.
So I'd propose there's a criteria to the effect of:
Significant population
Self sustaining (more or less)
Economically viable
Imperial recognition as a independent sovereign world (probably the most important criteria)
Not all worlds are necessarily habitable, an Imperial world where everyone is underground or in domes is certainly possible, if it has sufficient importance and population.
There should be only one Governor per world (though multiple claimants and civil wars are always possible), recognized by Holy Terra. There may be more than one per star system (if there are multiple inhabited worlds), and maybe there are sufficiently large and important space stations and asteroids recognized as 'worlds'. There may be all sorts of outliers too - A research outpost whose head somehow convinced the Imperium to recognize him as governor of 3 domes and 100 personnel, a clerical error recognizes Ceti Alpha 9's mine foreman as a governor instead of the Supreme Arch Baron of the Hive World Ceti 9 Alpha, stuff like that.
This becomes a bit of circular definition, if a planet has a recognized governor it's a world, if it doesn't it's not.
I like to think that lost worlds are also counted. Once the Imperium has recognized a planet as their, it's there's forever. An ork invasion/Tyranid harvest/daemon outbreak or exterminus doesn't negate the Emperor's divine right to that world.
Also I think the Imperium has a religious mandate to spread humanity to as many worlds as possible, reclaiming lost human worlds, terraforming potential worlds, or scratching an existence out of some marginal world. Until a world is recognized it would be an outpost, answering to the Imperial Governor or other government arm that settled it.
So the million number might be far too low (if you count outposts, colonies, settlements) and too high (since every Tyranid eaten world is still counted).
Sound about right?