1. an error in reasoning or argument that leads to a conclusion that may appear valid but is actually invalid. A fallacy may be formal or informal. A formal fallacy involves a violation of a principle of formal logic. An informal fallacy leads to an invalid conclusion because it is misleading in its language or appears to apply to a situation when it does not really apply. See also either–or fallacy; false analogy; false authority; false cause; historical fallacy.

2. more generally, any mistaken idea. —fallacious adj.