Meaning
M is a numeral letter, and among the ancients stood for a thousand; a use which is retained by the moderns. With a dash or stroke over it, it stands for a thousand times a thousand, or a million.
As an abbreviation, M stands for Marcus, Martius, Manlius or Mutius.
A.M. or M.A. stands for artium magister, master of arts; M.D. for medicinoe doctor, doctor of medicine; A.M. for anno mundi, the year of the world; MS, for manuscript; MSS, for manuscripts.
In astronomical tables, M stands for meridian, meridional, or mid-day.
In medical prescriptions, M stands for maniple, or handful, or misce, mix, or mixtura, a mixture.
In the late British Pharmacopaeias it signifies mensura, by measure.
In law, M is a brand or stigma impressed on one convicted of manslaughter, and admitted to the benefit of clergy.