= Planck constant
{c}
{title2=$h$}
{title2=6.62E-34}
{wiki}
Proportionality factor in the <Planck-Einstein relation> between light energy and frequency.
And analogously for matter, appears in the <de Broglie relations> relating momentum and frequency. Also appears in the <Schrödinger equation>, basically as a consequence/cause of the de Broglie relations most likely.
Intuitively, the <Planck constant> determines at what length scale do quantum effects start to show up for a given energy scale. It is because the Plank constant is very small that we don't perceive quantum effects on everyday energy/length/time scales. On the $\lim_{h \to 0}$, quantum mechanics disappears entirely.
A very direct way of thinking about it is to think about what would happen in a <double-slit experiment>. TODO think more clearly what happens there.
Defined exactly in the <2019 redefinition of the SI base units> to:
$$
6.62607015 \times 10^{-34} J \cdot s
$$