Yet again, with this UK election, I keep on seing people, both in online discussions, and in the press and TV, airily talking about "majorities" in cases where there is not any majority.

What they are talking about is the situation where one party, or one person, gets more votes, or more seats, than any other. But typically the winner will in fact have got less than half the total votes, or seats. The word for that is not "majority" but "plurality".

Is it pedantic to point out that difference? Has the language undergone the same kind of erosion under which "decimate" is so generally taken to mean almost annihilate, rather than to kill one in ten?

I think preserving the meaning of majority is important, because when used in these cases it seems like a dishonest attempt to suggest that the winner has wider support than is actually the case.