How do you define democracy? Do you believe the United States is a Democracy?

I am taking political Science in college this semester, and my professor keeps saying that the United States is a democracy, it might be democratic now, but it was set up as a republic originally. Why has it changed? She (my professor) frustrates me to no end! How can you study and teach political science and not know the origins of your own country?!
—what I am asking:
Do think the US is currently a repubilc or a democracy?
Why do you think this?
How do you define Democracy?—
Maybe I am being old fashioned or something, I don’t know, but I pay to learn things that are true, not some normative garbage.