Equivocation is classified as both a formal and informal 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 is often confused with amphiboly; however, equivocation is ambiguity arising from the misleading use of a word and amphiboly is ambiguity arising from misleading use of punctuation or syntax.
This form of word play relies on two different words, that sound alike, being used in different senses, and where these different senses are not immediately apparent, but become obvious upon a moment's reflection; for example the contrast between birth and death, and birth and berth, and told and toll'd in Thomas Hood's account of the death of Ben the sailor (which took place at the age of 40, contrasted with his age of zero at birth) in his humorous poem Faithless Sally Brown:
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:
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 is not true that all feathers are "bright"), this type of equivocation is actually an example of the fallacy of four terms.
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 as they go in such a way to achieve equivocation by treating distinct meanings of the word as equivalent.
In English language, one equivocation is with the word "man", which can mean both "member of species Homo sapiens" and "male member of species Homo sapiens". A well-known equivocation is
where "man-eating" is taken as "devouring only male human beings".
A separate case of equivocation is metaphor:
Here the equivocation is the metaphorical use of "jackass" to imply a stupid or obnoxious person instead of a male donkey.
This occurs where the referent of a word or expression in a second sentence is different from that in the immediately preceding sentence; and, especially, where such a change in referent has not been clearly identified.
The source of this equivocation is what linguists term a switch-reference: where, the subject of the second statement is a different subject from that of the first -- as in the case of the homograph bank (which can just as easily designate a financial institution as a riparian zone).
The result is that, despite the application of two identical signs (i.e., the word "theory"), the actual referent of the word in the first statement, the actual object in the world which it designates, is an entirely different entity from the referent designated by the identical homograph in the second statement:
And, moreover, regardless of whether or not it actually is, or is not, the case, it is transparently clear that the allegation that "evolution has never been subjected to scientific scrutiny" is not supported by (and is not demonstrated per medium of) the argument that is displayed in the "Evolution is a theory; Theories are uncertain; Therefore evolution is uncertain" syllogism.
Another example of switch-referencing: