How many years are there in a millennium?

A millennium is defined as a period of one thousand years. The prefix "milli-" signifies a factor of one thousand, which is where the term originates. Therefore, when asking how many years are in a millennium, the correct and direct answer is indeed one thousand years. This understanding highlights the importance of the metric system in naming and quantifying periods of time, as terms like decade (10 years) and century (100 years) also utilize similar prefixes to represent their respective durations.