It's pronounced like "key," get it?
According to the official Kingdom Hearts blog, Message from the Kingdom, the free-to-play spinoff that had previously been referred to as Kingdom Hearts PC has been given an official name: Kingdom Hearts X.
It isn't the Roman numeral ten; it's the Greek letter chi. Keeping with Square Enix tradition of using wacky words, chi in the Kingdom Hearts universe uses the pronunciation "key," rather than the more common "kai."
Those who pre-order Kingdom Hearts 1.5 HD Remix will get some bonus items for Kingdom Hearts X. Perhaps keeping with the theme of Greek letters, Kingdom Hearts 1.5 HD Remix is set to release on March 14, otherwise known as Pi Day.