Phlebotinum is a device that is used to advance a plot, classically in the television industry, although it can also appear in books and films. By its nature, it is usually inexplicable and often magical, with no basis in reality. It exists solely to propel the plot forward without unnecessary fuss, ideally with a minimum of suspension of disbelief. As one might imagine, phlebotinum is especially common in science fiction and fantasy, where unusual plot devices can be more believable.
The term was coined by the Buffy the Vampire Slayer writing team, when the writers were working on an episode and having trouble getting it to advance. As they struggled, one of the writers shouted “don't touch the phlebotinum in the corner,” and the term was born. The Buffy series is somewhat famous for such devices, with a variety of mystical objects and events being used to propel the arc of the story.
One of the most famous examples is probably kryptonite, the mysterious substance which is harmful to Superman. Phlebotinum often pops up when writers need a way to push a story, and they want to avoid a complex circumlocution that may require several episodes to unfold. By throwing phlebotinum into the mix, they can advance the story rather than focusing on the details, keeping readers interested and giving their characters something new to work with and play against.
Because phlebotinum is often impossible to explain, it tends to look out of place in shows that are based on reality or factual events, because viewers expect to see clear, rational explanations for what they see on screen. However, sometimes real-world objects or concepts can fulfill this role; forensics shows, for example, use a number of neat tricks to solve crimes and advance their plots, creating a veneer of scientific respectability for their phlebotinum.
Integrating phlebotinum into a plot can be tricky. Writers generally don't want to make the use of a plot device overly obvious, as it detracts from the story, so they try to introduce these devices with care, creating a situation in which its inclusion is at least reasonably acceptable. Its excessive use is also ill-advised, as fans can start to get impatient with the constant use of plot devices to solve (or create) problems.