Bringing some thinkers to enlighten us:
I am interested on the topic because I read that the Founding Fathers understood their system as a Representative Republic but not a democracy, they understood the word democracy as we understand direct democracy nowadays, it was a negative word seen as something close to "anarchy". That is, what we understand by the word representative democracy they understood by the word representative republic [I don't know how much of this is accurate either].
When and how the meaning of the word democracy changed?
I'm going to play devil's advocate here and disagree. I think people's understanding of democracy is pretty much the same.
Most people understand the US for example isn't a "democracy". When you see on the news the complaints about "undermining democracy" keep in mind that many systems in the 21st century still have a democratic component to them. The way most people interact with their system of government is by democratic means (eg: voting) so this is why they'll often emphasize the democratic part.
Could you explain where and how you think it has changed?