Handwavium is a way of getting around a plot hole or is an over-clever technology in fiction, especially science fiction; the writer waves their hand and a plot device appears that solves the problem (often by breaking the laws of physics, but you're not supposed to look at it closely.)
Some handwaviums include:
etc. etc.
It is distinguished from unobtainium which is a material that actually exists, but is extremely difficult or ridiculously expensive to get hold of. A handwavium usually or often is completely impossible.
jessicapierce writes: It would be helpful if you'd include some sort of derivation of the term, how long it's been used, where it originated, etc. I know it's a real term (such as it is) - but it may look to readers like you're making it up.
Sorry, don't have a clue who coined it, although the earliest reference to it appears in the google USENET cache back in 1994; it's clearly made from handwave with an ium stuck on the end to make it sound like an element; which in a sense it is, it is an element of a fictional story. The term seems to have been around for quite a while; but hasn't seen enough usage to make it into the big English dictionaries.