a deceptive, misleading, or false notion, belief, etc.

That the world is flat was at one time a popular fallacy.

a misleading or unsound argument.

deceptive, misleading, or false nature; erroneousness.

Logic . any of various types of erroneous reasoning that render arguments logically unsound.

Obsolete . deception.

noun

certainty , evidence , fact , honesty , reality , right , surety , truth

noun

aberration , ambiguity , artifice , bias , casuistry , cavil , deceit , deception , deceptiveness , delusion , deviation , elusion , equivocation , erratum , erroneousness , error , evasion , falsehood , faultiness , flaw , heresy , illogicality , inconsistency , inexactness , invalidity , misapprehension , miscalculation , misconstrual , misinterpretation , mistake , non sequitur , notion , paradox , perversion , preconception , prejudice , quibbling , quirk , solecism , sophism , sophistry , speciousness , subterfuge , untruth , falseness , falsity , spuriousness , illusion , misconception , paralogism