Equivocation

Equivocation is an informal logical fallacy that occurs when some word or group of words is used either implicitly or explicitly in two different senses. In other words, it is the use in a syllogism of a term more than once, but giving the term a different meaning each time.

Fallacious reasoning
Jonathan Sarfati illustrates the ambiguous use that evolutionists use to the term evolution: It is vitally important that words such as "evolution" be used accurately and consistently...However, many evolutionary propagandists are guilty of the deceitful practice of equivocation, that is, switching the meaning of a single word (evolution) part way through an argument. A common tactic, "bait-and-switch," is simply to produce examples of change over time, call this "evolution," then imply that the GTE is thereby proven or even essential, and creation disproved.

Semantic shift
Dean Overman provides a good example of this case :
 * Only man is rational.
 * No woman is a man.
 * Therefore, no woman is rational.

Switch-referencing

 * Margarine is better than nothing.
 * Nothing is better than butter.
 * Therefore, margarine is better than butter.