Definition
Misinformation is false or misleading information that spreads regardless of whether anyone intends to deceive. The defining feature is the falsity of the content, not the motive of the people sharing it. A well-meaning person passing on a wrong fact is spreading misinformation just as surely as a deliberate fabricator.
This distinguishes it from disinformation, which is false information spread with intent to deceive. Misinformation is the broader and more common phenomenon, because most people who circulate falsehoods believe them to be true and are simply being helpful.
Why it matters
How it works
Misinformation spreads through ordinary, well-intentioned sharing. Someone hears a striking fact, finds it plausible or useful, and passes it on. Each share adds reach and the appearance of consensus without adding any verification, so the false claim accumulates social proof while remaining unchecked.
Because the spreaders are sincere, appeals to honesty or motive do not stop the flow. What stops it is a habit upstream of sharing: confirming the content against a reliable source first. The Book of Common Fallacies is in effect a record of misinformation that survived for exactly as long as no one checked it.