- 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 morgan5. see molal6. see Mach
Dictionary of units of measurement. 2015.