Definition of "mainstay"

At the other extreme, we have that true maid-of-all-work type, the 0-6-0, the mainstay of goods, shunting, and often of passenger work right from the earliest days to the present time.

1945 November and December, H. C. Casserley, “Random Reflections on British Locomotive Types—1”, in Railway Magazine, page 319

As with most other railways, freight revenue is the mainstay of the balance sheet. In Canada, passenger revenue is only about one-tenth that of freight.

1959 November, J. N. Westwood, “The Railways of Canada”, in Trains Illustrated, page 555

Oil is the mainstay of Nigeria's economy.

2000, Jedrzej George Frynas, Oil in Nigeria

Conventional radiography has a major role in, and remains the mainstay of, initial evaluation and follow-up of rheumatologic disease.

2014, Marc C. Hochberg, Alan J. Silman, Josef S. Smolen, Rheumatology, page 307

On the Bishops Stortford line, the crisis now seems to be over; the units designed for this service are the mainstay of the workings once again and although some of the inner suburban sets are still seen, very few L.T.S. Line units are noticeable.

1963 January, “Motive power miscellany”, in Modern Railways, page 65

Like show dogs, dog actors became a mainstay in European and American contexts from the early nineteenth century with the convergence of public sentiment for dogs and popular interest in training them.

2004, Susan McHugh, Dog

Crickets are a mainstay of panfishing with live bait—and a mainstay of bait shops—but they come off the hook easily and you'll be plagued by minnows and tiny fish constantly stealing your bait.

2010, Lamar Underwood, 1001 Fishing Tips

X-Men: Apocalypse, directed by series mainstay Bryan Singer, gives Magneto, the Holocaust survivor who can control magnetic fields, and Xavier, the paraplegic telepath who tends to come off as really smug, next-to-zero shared screen time.

2016 May 23, Ignatiy Vishnevetsky, “Apocalypse pits the strengths of the X-Men series against the weaknesses”, in The Onion AV Club‎

Lauren Hemp, making her 50th appearance for the Lionesses, was preferred in central attack over mainstay Russo, while Chelsea centre-back Jess Carter was the chosen replacement for injured club team-mate Millie Bright.

2023 December 1, Emma Sanders, “England 3-2 Netherlands”, in BBC Sport‎