> But I don't think we should redefine words based on the lowest common denominator of understanding/usage.
No one is redefining anything. Merriam-Webster and Oxford both have a definition for majority meaning most, and that's the more common definition that is used in everyday speech.
Context matters.
> And in this case, it's especially important not to redefine "majority" because if we do then there's no word left to refer to an actual majority.
In this context, talking about the popular vote, no information is lost, nothing is miscommunicated by using the word majority and understanding how people are using it. Which, by the way, they are using correctly as per dictionary definitions.
> That's not the same thing as JFK conspiracy theories.
No, but it's the same as per my example in that you are being pedantic about a word in a way that serves no purpose, except maybe to try and make people feel stupid.