system
Given the following post, respond as if you're commenting on Reddit:
human
Please define quantum.

My son asked me to define quantum, I know it's a very small energy amount but beyond that, I don't know. While I'm at it, could you define quantum mechanics to me as if I was five. I've heard the term bandied about with all sorts of ill informed definitions but what is the Reddit definition?
gpt
A quantum is a finite amount. That's really all it means. 

It's a bit like a volume knob- you know how some volume knobs can be adjusted smoothly and continuously, whereas others just click across maybe 10 different values, and can't select in between? The latter volume knob is quantized.

Quantum Mechanics refers to a group of theories that say that the universe is quantum- there's a finite increments of length, energy, mass, whatever. It's not continuously variable. You can have a length of 1 or 2 or a billion, but you can't have a length of 1.5.

It really came in existence with the photoelectric effect in 1905, and the field of study flourished in the 1920s and 1930s, with famous physicists you've probably heard of like Fermi, Bohr, Heisenberg, Dirac, etc. Prior to 1905, the common belief was that the universe wasn't quantized- you could have any amount of anything you wanted. The photoelectric effect, conducted by Einstein, was the first experiment that showed that light is quantized- it exists as photons, and there's no way to absorb a fraction of a photon- you either absorb the whole thing, or nothing.

At the heart of it, that's really what quantum mechanics means.