Equivocation exploits the ambiguity of language by changing the meaning of a word during the course of an argument and using the different meanings to support some conclusion. A word whose meaning is maintained throughout an argument is described as being used univocally. Consider the following argument: How can you be against faith when we take leaps of faith all the time, with friends and potential spouses and investments? Here, the meaning of the word “faith” is shifted from a spiritual belief in a creator to a risky undertaking.
A common invocation of this fallacy happens in discussions of science and religion, where the word “why” may be used in equivocal ways. In one context, it may be used as a word that seeks cause, which as it happens is the main driver of science, and in another it may be used as a word that seeks purpose and deals with morals and gaps, which science may well not have answers to. For example, one may argue: Science cannot tell us why things happen. Why do we exist? Why be moral? Thus, we need some other source to tell us why things happen.
Taken from An Illustrated Book of Bad Arguments.