Mainstay (noun)

1. A person or thing that provides support or stability for a particular group or organization.
2. A fundamental or essential element of something.

Origin:

The word "mainstay" originated in the 16th century from the combination of the words "main" and "stay", which refers to the main support or backbone of a structure or system.

Examples:

1. She has been a mainstay of the company for many years.
2. The company's mainstay product is its flagship software.
3. He is the mainstay of the team, always dependable and reliable.
4. The small business's mainstay customers are the local residents.
5. The mainstay of the economy is the tourism industry.