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Equivocation ("to call by the same name") is classified as an informal logical fallacy. It is the misleading use of a term with more than one meaning or sense (by glossing over which meaning is intended at a particular time). It generally occurs with polysemic words (words with multiple meanings).
It is often confused with amphibology (amphiboly) (ambiguous sentences.); however, equivocation is ambiguity arising from the misleading use of a word and amphiboly is ambiguity arising from the misleading use of punctuation or syntax.
Fallacious reasoning 
Equivocation is the use in a syllogism (a logical chain of reasoning) of a term several times, but giving the term a different meaning each time. For example:
- A feather is light.
- What is light cannot be dark.
- Therefore, a feather cannot be dark.
In this use of equivocation, the word "light" is first used as the opposite of "heavy", but then used as a synonym of "bright" (the fallacy usually becomes obvious as soon as one tries to translate this argument into another language). Because the "middle term" of this syllogism is not one term, but two separate ones masquerading as one (all feathers are indeed "not heavy", but it is not true that all feathers are "bright"), this type of equivocation is actually an example of the fallacy of four terms.
Semantic shift 
The fallacy of equivocation is often used with words that have a strong emotional content and many meanings. These meanings often coincide within proper context, but the fallacious arguer does a semantic shift, slowly changing the context by treating, as equivalent, distinct meanings of the term.
In English language, one equivocation is with the word "man", which can mean both "member of the species, Homo sapiens," and "male member of the species, Homo sapiens." The following sentence is a well-known equivocation:
- "Do women need to worry about man-eating sharks?", in which "man-eating" is construed to mean a shark that devours only male human beings.
This occurs where the referent of a word or expression in a second sentence is different from that in the immediately preceding sentence, especially where a change in referent has not been clearly identified.
- All jackasses have long ears.
- Carl is a jackass.
- Therefore, Carl has long ears.
Here the equivocation is the metaphorical use of "jackass" to imply a stupid or obnoxious person instead of a male donkey.
"Better than nothing" 
- Margarine is better than nothing.
- Nothing is better than butter.
- Therefore, margarine is better than butter.
Specific types of equivocation fallacies 
- See main articles: False attribution, Fallacy of quoting out of context, No true Scotsman, Shifting ground fallacy.
See also 
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- Logical Fallacy: Equivocation The Fallacy Files