莫耳（拉丁文「一团」）是物质的量的国际单位，符号为mol（mole）。1莫耳是所含基本微粒个数与12克的碳-12()中所含原子个数相等的一系统物质的量。使用莫耳时，应指明基本微粒，可以是分子、原子、离子、电子或其他基本微粒，也可以是基本微粒的特定组合体。1莫耳物质中所含基本微粒的个数等于阿伏伽德罗常数，符号为N_A，数值约是6.02214129×10，常取6.02×10。

一种物质的莫耳质量与式量，在使用国际单位制时，在数值上相等。

摩尔浓度的单位是摩尔每升，写作mol/L或mol·L，或简写为M。需要注意的是，1N＝（1mol/L）×粒子价数。

The mole is the unit of measurement in the International System of Units (SI) for amount of substance. It is defined as the amount of a chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12 (C), the isotope of carbon with relative atomic mass 12 by definition. This number is expressed by the Avogadro constant, which has a value of 7023602214085700000♠6.022140857(74)×10 mol. The mole is one of the base units of the SI, and has the unit symbol mol.