In brief, the same word is used with two different meanings. In other words: "The fallacy of Equivocation is a pervading and ubiquitous fallacy. It varies in subtlety from a manifest pun, that would not deceive a child, to a confusion the most subtle, the most difficult to detect, to recognise, and to avoid, of all fallacies; and of all fallacies it is the most frequently perpetrated. Nothing is more frequent in reasoning, and especially in disputation, than the use of a term in two or more senses, without any appreciation on the part of either of the disputants, or of the single reasoner, that it is used in more than one; nor is there any source so fertile of difference of opinion. In fact, difference, that appears to be difference of opinion about facts, is very often, unknown to the disputants, difference about the meaning of words; and no controversy can be useful or fertile, that is not preceded by a definition of the words to be used, and an agreement about the meanings to be attached to them. It is not too much to say, that in most controversies, each party uses some important term, on which the controversy hangs, in a sense different from that understood by the other party; or uses the term, first in one sense, and then in another, without any recognition or appreciation of the equivocation. It requires an effort, and a consider able effort, to adhere to the same meaning in using a word of current and large signification, throughout a controversy that is at all prolonged." (Mercier, A New Logic, pp. 366-7.)
Fallacies of Ambiguity
The fallacies in this section are all cases where a word or phrase is used unclearly. There are two ways in which this can occur. (1) The word or phrase may be ambiguous, in which case it has more than one distinct meaning. (2) The word or phrase may be vague, in which case it has no distinct meaning.
An amphiboly occurs when the construction of a sentence allows it to have two different meanings.
Emphasis is used to suggest a meaning different from the actual content of the proposition.