In Salman Rushdie's Haroun and the Sea of Stories "Kahani" is the name of earth's second moon. Kahani circles the earth at such high speed and at a constantly changing angle that it can't be detected by any of our instruments. Or as Butt the Hoopoe puts it:

The moon, Kahani, travels so fast - wonder of wonders! - that no Earth instruments can detect it; also its orbit varies by one degree per circuit, so that in three hundred and sixty orbits it has overflown every spot upon the Earth. Variety of Behaviour assists in Evasion of Detection. But also, there are serious purposes for this variation of orbit: Story Water facilities must be provided across the entire planet with an even hand.

Kahani is home to the "Ocean of the Streams of Story", that place where every story in the world originally comes from. Storytellers on earth have a subscription to the story water and are therefore capable of telling ever-amazing stories and tales.

Kahani is also home to the two cities of Chup and Gup and their respective inhabitants, Chupwalas and Guppees. It houses human-like folks like the scientific Eggheads who run all the P2C2Es, the king of Gup and his army consisting of pages, water genies who maintain the story water supplies of subscribers; but also floating gardeners who take care of the different streams of stories in the Ocean of the Streams of Stories, and Plentimaw Fishes who only appear in pairs.

To tell more about the moon Kahani would be to spoil the story - read the book if you want to know more.

Kahani also turns out to be the long forgotten name of spoiler!

Kahānī (as the "proper" romanization should be (with macrons above the a and the i)) is a Hindi word originally, but also used in Urdu since they really are the same language. It means "story", "fable" or "tale". For those of you whose browsers can read unicode, here are the pretty original versions of Kahani:

This is Hindi in Devanagari script:

This is Urdu in Arabic script:

Thanks go to tres equis for providing me with the correct Urdu and Hindi spelling plus unicode and a translation :)