The letter "M" can stand for different things depending on the context. Primarily, in Roman numerals, "M" represents the number 1000. In finance and accounting, "M" is often used to indicate thousands, while "MM" is used to denote millions (1,000 times 1,000). This usage comes from Roman numeral tradition where "M" means thousand and doubling it (MM) means a thousand thousands, or one million. Additionally, "M" is the unit prefix "mega" in the metric system, meaning one million times (10^6). It is also commonly used as an abbreviation for medium (such as clothing size), meter (the SI unit of length), male gender, molarity in chemistry, and other specific terms in different fields. Overall, the specific meaning of "M" depends heavily on the subject area: Roman numerals, finance, science, or general abbreviations.