"On hold... indefinitely"
After the runaway success of the Starcraft real-time strategy game, Blizzard Entertainment began production on StarCraft: Ghost, a spin-off, which would have been a third-person action stealth game in the vein of the then-popular Metal Gear Solid and Splinter Cell franchises, starring a ghost operative named Nova. However, the game was repeatedly delayed until Nihilistic Software, who had been brought in specifically for work on Ghost, discontinued working on the project. Blizzard then continued working on the project in collaboration with Swingin' Ape Studios and eventually bought the company in May 2005.
StarCraft: Ghost is officially "on hold" according to Blizzard representatives, rendering it one of the longest-running vapourware titles in gaming history, but the game is generally considered cancelled. Blizzard Entertainment has a history of cancelling near-completed projects in order to maintain their standard of releasing extremely high quality games. It's commonly accepted amongst the press and the public alike that Ghost was cancelled for that very reason.
Interestingly, shortly after the "on hold" announcement was made, Blizzard released a new novel, "Starcraft: Ghost - Nova", which featured a storyline starring Nova, the Ghost operative who was meant to be the player character in Ghost. This action has been compared to similar actions taken by Blizzard after the cancellation of the adventure game Warcraft Adventures: Lord of the Clans, specifically that they announced that even though the public would never get to play the game, its storyline was still Warcraft canon, and so they released a novel, "Warcraft: Lord of the Clans", to share that story with their fans.
The game struggled through a lengthy development cycle, often re-creating itself in order to live up to new standards being set by other games. Shane Bettenhausen from 1UP.com spoke of Starcraft: Ghost, several months after it had been cancelled on the weekly 1Up Yours show. In this show, Bettenhausen theorized that Ghost had attempted to become everything that was popular at the time, taking aspects from games like Battlefield 2 for its multiplayer, and Mercenaries: Playground of Destruction in combination with Splinter Cell for the singleplayer experience. Because of this constant identity crisis, the game could never be "finished" and was "put on hold." While this PR-friendly (though not particularly Blizzard-friendly) explanation holds some water, the truth may never be known.
Proving that at least some of the storyline was meant to be canon, Nova and General Warfield, characters meant to be in Ghost, made brief appearances in StarCraft II: Wings of Liberty .