Molar (symbol: M) is a unit of concentration that measures the amount of a substance per unit of volume. Specifically, molar concentration represents the number of moles of solute (a substance being dissolved) per liter of solution.

One mole is defined as the amount of a substance that contains the same number of entities (such as atoms, molecules, or ions) as there are atoms in exactly 12 grams of pure carbon-12. The number of entities in one mole is called Avogadro's number, which is approximately 6.022 x 10^23.

For example, a solution with a molar concentration of 1 M contains one mole of solute per liter of solution. This means that there are 6.022 x 10^23 molecules (or other entities) of the solute present in each liter of solution.

Molar concentration is commonly used in chemistry, biochemistry, and other scientific fields to express the concentration of substances in a solution. It is particularly useful for measuring the concentration of ions in a solution, such as in acid-base reactions.

To convert between molar concentration and other concentration units, such as millimolar (mM) or micromolar (μM), conversion factors must be used. For example, one millimole (mmol) of solute per liter of solution is equivalent to 1 mM, and one micromole (μmol) per liter is equivalent to 1 μM. Conversely, one M is equal to 1000 mM or 1,000,000 μM.