The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
There's also Unobtainium and other stuff under Macguffin.
Applied Phlebotinum is where we store the lengthy list of tropes about this (including Unobtanium, Aesoptinum, It Runs on Nonsensoleum, Minovsky Physics and much more). The term "phlebotinum" is our general default, and there are dozens of tropes with the word in their name (Phlebotinum Battery, Phlebotinum-Induced Stupidity, Imported Alien Phlebotinum, etc.)
Edited by Xtifr Speaking words of fandom: let it squee, let it squee.
See also Plot Device, for when it exists to allow a story to be told.
What are the popular terms for plot facilitating technical, engineering, or scientific constructs? I personally use the term 'flumtech.'