What does the unit M stand for? [closed]

What does $\mathrm{M}$ signify in this pH calculation? To me, it just seems to be a random variable.

For example: $\ce{[H+]} = 1.0\times10^{-7}~\mathrm{M}$

I am not sure what the $\mathrm{M}$ is supposed to mean?

closed as off-topic by Tyberius, A.K., Waylander, Jon Custer, airhuffOct 12 '18 at 23:57

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• M = Molarity = mol per Liter of solution – A.K. Oct 11 '18 at 16:04

Amount of a constituent divided by the volume of the mixture. Also called amount-of-substance concentration, substance concentration (in clinical chemistry) and in older literature molarity. For entities B it is often denoted by $\ce{[B]}$. The common unit is mole per cubic decimetre ($\mathrm{mol\, dm^{-3}}$) or mole per litre ($\mathrm{mol\, L^{-1}}$) sometimes denoted by $M$.
In your example it just means the concentration of protons in solution is $\ce{[H+]} = 1.0\times 10^{-7}~\mathrm{mol/ l}$.