the mole
A counting unit for atoms or molecules, like a dozen means 12, only enormously larger. Particles are far too tiny to count one by one, so chemists count them indirectly by weighing; a mole is one specific, huge quantity of them.
Precisely, one mole is exactly 6.02214076 x 10^23 elementary entities, a value known as Avogadro's number. The number is chosen so that one mole of a substance has a mass in grams equal to its atomic or molecular mass in atomic mass units: 1 mole of carbon-12 is 12 g, and 1 mole of water is 18 g.
Why: the mole bridges the microscopic world of atoms and the macroscopic world of grams you can weigh on a balance. In P V = n R T, the n is measured in moles. Since 2019 the mole has been defined directly by fixing Avogadro's number at that exact value.
18 grams of water is 1 mole, containing about 6.022 x 10^23 water molecules.
The mole is a counting unit linking atoms to grams.
A mole counts elementary entities, so always state whether you mean atoms, molecules, or ions to avoid confusion.