M

   1. an informal abbreviation for million in expressions such as "$500M" for 500 million dollars or "Unemployment Reaches 4M" in a newspaper headline. In binary contexts such as computer memory, M often represents 2²⁰ = 1 048 576 (see mebi-, below).

   2. the Roman numeral 1000, sometimes used in symbols to indicate a thousand, as in Mcf, a traditional symbol for 1000 cubic feet. Given the widespread use of M to mean one million, this older use of M to mean one thousand is very confusing and should be scrapped.

   3. the symbol for "molar" in chemistry.

   4. see morgan

   5. see molal

   6. see Mach