The prefix for one million is "mega-." In the metric system, prefixes are used to denote multiples or fractions of units, and "mega" specifically refers to a factor of one million (10^6). For example, one megabyte (MB) equals one million bytes, and one megawatt (MW) equals one million watts. The use of prefixes like mega facilitates scientific communication by simplifying the expression of large numbers, making it easier to understand and compare quantities across various fields, including science, technology, and engineering.