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mM stands for millimolar, a unit of concentration used in chemistry, biology, and medicine. It denotes the amount of solute per liter as one thousandth of a mole, written as 10^-3 mol/L. Consequently, 1 mM equals 0.001 M, and 1 M equals 1000 mM. The unit is commonly used to express concentrations of solutes in aqueous solutions, especially when reactants and products occur at millimolar levels.

Applications and contexts: mM is widely used to describe the concentrations of buffers, substrates, cofactors, and drug solutions in biochemistry, molecular biology, physiology, and pharmacology. It provides a convenient scale for reactions and experiments that operate in the millimolar range. Preparation of solutions is typically done by diluting a stock solution; for example, from a 1 M stock, a 0.01 M (10 mM) solution can be prepared by a 100-fold dilution.

Related units and notation: 1 mM equals 1000 µM, and 1 mmol/L is an equivalent notation (since 1 mmol/L = 1 mM). The uppercase M denotes molarity (moles per liter); the prefix m stands for milli. The unit should not be confused with millimeters (mm). In practice, researchers may also express concentrations in M, mM, or mmol/L depending on context and clarity.