A justified Trope is one in which, in its common usage, runs counter to the normal laws of logic and probability but in a particular instance has a concrete reason for applying to the story. In other words, a trope is "justified" if it is required by other pre-existing elements in the story—otherwise the explanation is a Hand Wave.
Note that there are varying degrees of justifying a trope and a spectrum of justification from the completely logical to A Wizard Did It. For a justification to be more than just A Wizard Did It, there has to be reasons that make it make sense in context.
Here's a couple example scenarios:
The justified case:
- The protagonist uses a glowy sword because he fights sentient shadows.
- The car blew up upon crashing because of the nitroglycerin that was put in the trunk in an earlier episode as part of a ploy to blast through a bank vault.
- The protagonist uses a glowy sword because the power source is leaky (but wait, shouldn't that cause other problems? We also know that he has the tools to fix it!).
- The car blew up upon being shot because the shooter aimed for the gas tank and said he used tracer rounds (but wait, why would he have tracer rounds in his service pistol?).
A Justified Trope has common ground with Lampshading in that both often deal with tropes that are typically illogical or otherwise problematic.
Remember that a trope being justified is not about it ultimately being good or bad or effective or ineffective - it's about whether or not it makes sense from an in-universe point of view. A trope can be perfectly justified and still ruin a show. Contrarily, a trope may be completely unjustified and make the story better. Do not assume that the existence of a trope is inherently a sign of bad writing.
See The Watson, whose job is to explain justifications, and Meta Trope Intro which compares this with many other ways that a trope can be used. If the Justified Trope falls short, it's a Hand Wave. If the justification just makes the illogic of the trope worse, then it becomes a Voodoo Shark.
One thing to note, if you think a trope is "justified", you should never use a Justifying Edit. If you were Pot Holed here by a Justifying Edit, integrate the potholed text to the main text or delete it outright. See Justifying Edit for more on this.