Using a word with more different meanings during the course of an argument.
Changing the meaning of a word during an argument.
Treating two different concepts as equivalent on the basis of the fact they're both designed by the same word.

General form:
"A is defined as B, and also as C, therefore P(B) is the same as P(C)"


Typical example:

Logic revolves around concepts, not around words; for this reason, words must be defined unambiguously beforehands, and their meaning can never be changed; and in cases where a word has multiple meanings, it should always be specified which one one is referring to, in order to avoid ambiguity.