Selective redefinition is the ad-hoc alteration of the meaning of a word in order to selectively include examples with the same "charge" and exclude examples with the opposite one;
in the case of a word with a strongly positive acception, whenever something that would normally fall under the scope of that word becomes recognized as negative (i.e.: false and/or harmful), the definition of the word is altered on the spot in order to exclude that example; instead, whenever something that would normally fall outside the scope of that word becomes recognized as positive (i.e.: true and/or useful), the definition is altered in order to include that example.
On the contrary, in the case of a word with a strongly negative acception, the definition is altered in order to exclude positive examples and include negative ones.
In this way mainstream media can ensure that said words will never lose their manipulatory power for propaganda purposes, and the narrative can be kept intact in spite of any contrary evidence.
A typical example of the first case is the word "science": normally, the word is used in such a way to arbitrarily exclude anything that might damage corporations' profits or the reputation of the institutions, regardless of any evidence in favor; however, whenever the evidences in favor of an "unscientific" theory become too widely recognized to be denied without losing even more credibility, they are suddenly taken into account again, so that idea becomes accepted into the realm of "science"; this also helps to hide the dogmatic nature of mainstream "science" and give the illusion of an open, evolving science able to perfect itself and learn from its mistakes, with the paradoxical effect that the more it is proven wrong, the more credibility it gains.
A typical example of the opposite case is the term "conspiracy theory": because of its negative acception, it is commonly used by mass-media and propagandist to include anything that might damage corporations' profits or the institutions' credibility, again regardless of any evidence in favor; but whenever the evidences of a conspiracy become too strong to be hidden anymore, or their supporters become too numerous and vocal to be countered, that particular conspiracy stops being considered a "conspiracy theory" and instead becomes a "historical fact"; in this way it is possible for conspiracy deniers to keep their narrative intact even after repeated falsifications, and to ensure that the derogatory power of the term "conspiracy theory" is left untouched.