An equivocation fallacy is committed when the same word is used in different contexts (or with differing meanings) in different parts of the same argument. A trivial example would be arguing that because a road sign stated “fine for parking here”, that therefore it was acceptable to park in this location. In this case it is obvious that the word ‘fine’ has been used in the first case to mean ‘a monetary penalty’, and in the second instance to mean ‘permissible’.
Real world case of equivocation are typically far less obvious than this. Generally, they occur when a particular term has, or may have, slightly different meanings in different contexts, but in which these contexts are mixed or interchanged in an unclear way so as to make the argument sound more persuasive. An example of this might be the argument: “fetuses are human beings, and all human beings have the right to life, so all fetuses have the right to life.”
In this case, the use of the word ‘human being’ in the first case has a biological context, in which it is undeniable that a human fetus is biologically human. The second usage, however, seems to be appealing to a legal or moral context of the term ‘human being’. As such, it is not clear that the term means exactly the same thing in both clauses, since (at least potentially) it is not the case that everything that is biologically human is also a ‘human being’ in a moral and legal perspective (or visa-versa). If this analysis is correct, then this argument is using the term ‘human being’ differently in the two premises, and therefore the conclusion cannot be validly inferred from these premises alone.
Equivocation fallacies commonly involve use of value-laden words which can vary in meaning depending on context. Examples of such words include ‘right’, ‘innocent’, ‘justify’, ‘fair’, ‘equal’, ‘excessive’, ‘sufficient’, etc. Such potentially ambiguous terms should generally be clearly defined when used in an argument, so as to avoid unintentional equivocation.
Fallacies of ambiguity: overview from PhilosophyPages
Equivocation fallacy: overview from Logical Fallacies
Equivocation: overview from Fallacy Files