Definitions of M:

noun: the 13th letter of the Roman alphabet
noun: concentration measured by the number of moles of solute per liter of solvent
noun: the basic unit of length adopted under the Systeme International d'Unites (approximately 1.094 yards)
noun: the cardinal number that is the product of 10 and 100
adjective: denoting a quantity consisting of 1,000 items or units