Avogadro's number
/ ah-vuh-GAH-droh /
The huge number of particles in one mole, about 6.022 x 10^23, so large it is hard to picture: a 6 followed by 23 more digits.
Precisely, N_A = 6.02214076 x 10^23 per mole, an exact defined value since 2019. Written as the Avogadro constant it carries units of 1/mol. The name honours Amedeo Avogadro, whose hypothesis that equal volumes of gas at the same temperature and pressure contain equal numbers of molecules underlies the idea, even though he never measured the number himself.
It links the universal gas constant R and the Boltzmann constant k_B through R = N_A k_B, connecting the per-mole and per-molecule ways of doing gas calculations. For a sense of scale, a mole of marbles would bury the entire surface of the Earth many kilometres deep.
The relation R = N_A k_B, between the universal gas constant and the Boltzmann constant, runs through this number.
The number of particles in one mole, about 6.022 x 10^23.
Avogadro's number is just a pure count; the Avogadro constant is the same value but with units of per mole, which is what you use in equations.