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.
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"), equivocation is actually a kind 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 ass.
In the first statement, "nothing" really means "dry bread" (such that the sentence means "it is preferable to have margarine [on bread] than nothing at all"), whereas in the second, it means, literally, "no thing" (so the sentence means "there exists no thing that is better than butter").