Death really isn't the end in science fiction... It just depends on whether or not it can be written around later. Here are some of our favorite NotDeaths that prove that the Grim Reaper should really up his game.

Died: Sacrificing himself by bringing the warp engines back online at the end of Star Trek II: The Wrath of Khan, leading to his dying from exposure to radiation.
Undied: His body was resurrected in Star Trek III: The Search For Spock thanks to the Deus Ex Machina powers of the Genesis Planet, and it turned out that his soul had lived on all along thanks to mind melding with Bones.
Cause of Undeath: Mind-meld and blatant plot ridiculousness in order to keep the fans happy. Admittedly, it was all set up in Star Trek II, but still.
Does It Count As Death?: Well, his soul was alive the entire time in Bones, but his body had enough time to go through a funeral and being shot off into space, so... 50/50? But not really, let's face it.

Ellen Tigh
Died: Poisoned by her husband after (in his eyes) betraying humanity in "Exodus, Part II" at the start of Battlestar Galactica's third season.
Undied: Instantly downloaded into a new body as part of the Fifth Cylon retcon, as revealed in the fourth season's "Sometimes A Great Notion."
Cause of Undeath: Traditional cylon download/rebirth.
Does It Count As Death?: Well, she was instantly reborn, which suggests that she was never actually dead as such, but the whole Fifth Cylon thing muddies the waters... especially when she was reborn as someone who wasn't exactly the Ellen she was when she died. We're going with "Kinda, but not really."

Boba Fett
Died: Falling into the Sarlacc's mouth in Return Of The Jedi.
Undied: Climbing back out of the Sarlacc's mouth in comic sequel Star Wars: Dark Empire.
Cause of Undeath: He was swallowed by apparently never chewed or digested and climbed his way out, apparently.
Does It Count As Death?: If you believe Dark Empire, not in the slightest. George Lucas apparently disagrees, however; it's said that he edited Fett's last appearance in the special edition of Return Of The Jedi to make it clearer that it's meant to be the end of the character.

John Sheridan
Died: Avoiding certain death by nuclear explosion at the end of Babylon 5's third season finale, "Z'ha'dum," by jumping into a pit so deep that it was impossible to survive. Oh, and then there was that nuclear explosion, which presumably would've destroyed the pit and everything within it anyway.
Undied: At the start of the show's fourth season, Sheridan was revealed to be in a limbo between life and death because of his love for Delenn. With the help of - and 20 years worth of lifeforce from - helpful fellow limbo-ite Lorien, he comes back to the land of the living.
Cause of Undeath: As Ewan McGregor in Trainspotting would say, choosing life. Who knew it was that simple?
Does It Count As Death?: Nope. Think of it as getting as far as death's foyer, before deciding to turn back because you'd changed your mind.

Tasha Yar
Died: Wanting out of her Starfleet contract early, Denise Crosby got her character killed at the hands of a gloopy, ooky oil monster in the first season episode of Star Trek: The Next Generation "Skin of Evil."
Undied: Thanks to time travel shenanigans, turns out never to have died in the alternate timeline of third season episode "Yesterday's Enterprise," and then manages to return to the past of the original timeline at the end of the episode in a way that still doesn't make a lot of sense.
Cause of Undeath: Alternate timelines having prevented her from dying in the first place.
Does It Count As Death?: Well, a Tasha Yar definitely died. In fact, as we learn upon the appearance of the second Yar's daughter Sela, the other Tasha was killed unsuccessfully trying to escape from the Romulans, so it looks as if any and all Tashas would end up dead one way or another.

Died: At the hands of the apparently unstoppable Doomsday in 1993's The Death of Superman storyline.
Undied: Midway through the follow-on The Return of Superman storyline, when it's been revealed that none of the four characters who've taken up the mantle are the real thing.
Cause of Undeath: He woke up. No, really; the audience is pretty much told that he'd never died in the first place, he'd just gone into superhibernation in order to heal from the fight.
Does It Count As Death?: Not at all, but it definitely counted as a moneyspinner for DC Comics, who went on to kill Green Arrow and Green Lantern within the next couple of years, as well as teasing deaths for the Flash and breaking Batman's back.

Died: Trapped on a bomb that mentor and Nazi-fighting partner Captain America had managed to jump off of before it exploded, as explained way back in 1963's Avengers series.
Undied: In 2005's "Winter Soldier" storyline of Captain America, where he got reintroduced and prepped to become the new Captain America in 2007.
Cause of Undeath: Turns out that Bucky was, in fact, blown to bits by the exploding bomb... It's just that they were pretty large bits. Large enough to rebuild him into a brainwashed no-good commie assassin who gets put on ice between missions, until he meets Cap, goes rogue, remembers who he is, and then uses his mighty Russian technology for the good of American mankind.
Does It Count As Death?: What's brainwashed Russian assassin for no?

The Flash
Died: Which one? Barry Allen died in 1985's Crisis On Infinite Earths. Wally West disappeared and was, at various times, presumed dead/missing/no-one could make up their mind in 2004's Infinite Crisis, and Bart Allen kicked the bucket in 2007's The Flash: The Fastest Man Alive #13.
Undied: Wally came back in 2007's Justice League of America #10, Barry in 2008's Final Crisis #1 and Bart in 2009's Final Crisis: Legion of Three Worlds #4.
Cause of Undeath: Both Barry and Wally had, it turns out, never died. Barry had been swallowed into the Speed Force, which is the cosmic... thing... that gives all super-speed characters their powers in the DC Universe, while Wally's fate was ultimately (after a couple of failed attempts that were quickly contradicted) decided upon a variation of "He took his family on vacation to an alien planet and didn't tell anyone." Don't ask. Bart, meanwhile, did die, kind of... but his teenage self was trapped in a futuristic lightning rod and then magically released in the 31st century to fight Superboy Prime. Again, it's probably better if you didn't ask.
Does It Count As Death?: No question for either Barry or Wally (No), but Bart... I have no idea. I've read Legion of Three Worlds multiple times, and still don't understand the explanation that's given there; let's just never mention it again and pretend it didn't happen.

Jason Todd
Died: As the result of a real-life phone vote to see if Todd, the second Robin (as in Batman and), should be killed at the hands of the Joker. Seriously, 1988's comic industry, what the hell were you thinking?
Undied: 2004's Batman revealed that Todd was not only not dead, but had magically aged more than most other characters in the DC Universe in his off-panel absence.
Cause of Undeath: Superboy was punching the walls of reality, and things went a bit weird. You know how it is with these superheroes and their punching the walls of reality; history gets rewritten all over the place. Just be glad that Batman didn't end up as Batdinosaur. Although, now that we think about it, that'd be awesome.
Does It Count As Death?: Magically contradicting Schrodinger and his cat, Jason Todd both did and didn't die. His official history has it that he died, and then just came back to life thanks to the punching of reality, meaning that he was still alive. So, while it ultimately doesn't count as permanent death, there was a death in there somewhere.

Jean Grey
Died: In 1980's famous Uncanny X-Men #137, where she sacrifices herself for the good of the universe to stop herself from becoming overwhelmed by the godlike power she possessed that might lead her to eat a couple of planets if she got peckish.
Undied: It's revealed in 1986's Fantastic Four #286 that the Jean Grey who killed herself was never actually Jean Grey at all, but the Phoenix force, who's been cosmically imprinted with Jean's personality. Don't worry; the Phoenix force was already back by that point anyway.
Cause of Undeath: Jean hadn't died (at that point), and the resurrection of the Phoenix force was somewhat implied by the name - The official explanation was that the Phoenix force hadn't actually died either, just lain dormant until someone else (Jean's daughter from an alternate timeline. If you don't already know, don't ask) claimed it.
Does It Count As Death?: Before the retcon and ruined Chris Claremont's X-Men once and for all you bastards, it did. Now? No-one died until years later, when Jean really got the Phoenix power and then ended up dying anyway. Guess there's something unlucky about the name or something.