2 years ago
Last edited at 3:21PM on 1/14/2012
If it is a prefix it means "mega" which is Greek for a million: MHz is a mega hertz, MeV is mega electron volts, Mpc is mega parsec, etc. I don't think you meant a meter, because meter is abbreviated conventionally to lower case "m", not upper case.
The measurement M means different units such as an informal abbreviation for million in expressions, the symbol for "molar" in chemistry, symbol for "meters" in oceanography and the Roman numeral 1000 used in symbols to indicate a thousand among others.