A more thorough explanation:

Definition: A major is a commissioned officer in the U.S. Army, U.S. Air Force, or U.S. Marine Corps who ranks above a captain and below a lieutenant-colonel. In Roman law, a major was an older person, especially one older than 25 and hence of full capacity. It could also refer to an ascendant or ancestor. In history, a major was a mayor. For adults, major means something important or significant.

• After serving as a captain for several years, John was promoted to major in the U.S. Army.
• In Roman law, a major was considered to be an adult who had reached full capacity.
• My great-great-grandfather was a major in the Civil War.
• The construction of the new highway was a major undertaking for the city.

The examples illustrate the different meanings of the word "major." The first example shows how it is used in the military context, while the second example refers to its use in Roman law. The third example is a historical reference to a person's rank, and the fourth example demonstrates how it can be used to describe something significant or important.