The term "infodump" describes the conspicuous passing-on of information directly to the reader, typically within the main body of the text. Thus a footnote/endnote doesn't qualify, nor does a synopsis in a prologue (such as the scrolling text at the beginning of the Star Wars films).
An example would be a sudden lengthy digression into the backstory behind the current goings on.
Dirk Zaxxon eased back on the throttle coasted into the upper atmosphere high over Krenulak. The planet's tranquil appearance belied the thousand decades of bloody war that had once raged over its surface, culminating in the destruction of every major city and plunging the entire world into a new dark age. In the years that followed, countless petty baronies had risen and fallen, none managing to claim more than a few hundred square miles of blighted land before vanishing without trace. A civilization once hailed as the shining star of the galaxy had degenerated into warring clans savaging the corpse of the world like dogs devouring their slain master. Only direct intervention of the Super Star Squadron had kept the Krenulakians from destroying the last of the most ancient strongholds still rumored to house the secret knowledge of the planet's founders. Knowledge that might now hold the key to defending the Squadron from the marauding fleet threatening to extinguish all freedom. Nothing was known of the mysterious fleet, save that it had originated in the darkest regions of intergalactic space, and no force yet brought to bear had even slowed its advance.
Blah blah blah blah blah...
Sure, this information might be useful as backstory, but I hate when it's deployed in this fashion, as if the author is saying
"I should have told you this stuff earlier, but now you really need to know it before you read any further." I even accept that many people don't mind this kind of expository digression, but it usually seems like a crutch intended to prop up weak writing. Or else the author has spent a lot of time devising a universe, and she's determined to tell you all about it, no matter how frankly irrelevant it is to the actual story at hand.
Another example, on a smaller scale, is when a character uses dialogue to reveal information that should already be obvious to the participants in the conversation:
Infodump:
"Do you think he forgot?" asked Joe.
"Forgot what?" Mary sneered. "How we agreed to meet here tonight to discuss our plan for robbing the First Bank of Punxsutawney during the commotion of Groundhog Day? We swore to enact that plan two years ago, that night in the diner. How could he forget?"
Mary is telling Joe things that he clearly knows, since he's here for the meeting. Sure, in real life, people speak the obvious all the time, but dialogue isn't speech--it's a synthesis of speech. IMO dialogue should be used as a means of giving exposition only
very sparingly. Again, YMMV.
Not an infodump:
"Do you think he forgot?" asked Joe
Mary shook her head, recalling the agreement they'd made two years before.
Further description of the agreement and what it entails can follow in additional exposition, but Mary is freed of the dead weight of backstory-dialogue.
I admit also that this is all a
hugely subjective judgment call--some readers enjoy reading all about the history of a story's setting. I also realize that sometimes an infodump can be used skillfully and with great effectiveness while not disrupting the pace of the story.
I guess that's the purpose of the poll!