quantum
/ KWON-tum /
A quantum is the smallest possible amount of something — the tiniest "chunk" that can exist, with nothing smaller in between. Energy, for example, doesn't flow like water from a tap; it comes in discrete little packets, like coins of a fixed size rather than an endless smooth stream.
The idea was born in 1900 when physicist Max Planck realized he could only explain how hot objects glow if energy came in these set-sized packets, not in any amount you please. That single insight cracked open a whole new science — quantum physics — describing how the universe behaves at its smallest scales.
When you have more than one, you say "quanta." So a beam of light is made of countless quanta of energy, each one indivisible — you can have one or two, but never half.
The word comes from the Latin "quantus," meaning "how much." So a "quantum leap," despite sounding huge in everyday talk, literally describes the tiniest jump nature allows.