Accidental and Incidental

The adjectives accidental and incidental sound similar but have different meanings.

Definitions

The adjective accidental means unintentional or happening by chance.

The adjective incidental means secondary or nonessential. It often refers to something that occurs in connection with a more important activity or event.

Examples

• According to the National Safety Council, there were more than 600 accidental shooting deaths in the United States last year.
• "We can reduce the risk of setting off accidental nuclear war by retiring nuclear cruise missiles and instead rely on conventional weapons."
  (Dianne Feinstein and Ellen O. Tauscher, "A Nuclear Weapon That America Doesn't Need." The New York Times, June 17, 2016)  
• "Traditionally, vocabulary learning was often left to look after itself and received only incidental attention in many textbooks and language programs."
  (J.C. Richards and W.A. Renandya, Methodology in Language Teaching. Cambridge University Press, 2002) 
• "We had to save everything we could against the worst possibilities. As for incidentals, they were going to be very incidental indeed. Scratch those too."
  (Wallace Stegner, Crossing to Safety, 1987) 

Usage Notes

"What is accidental happens by chance: 'We didn't plan our meeting at the restaurant; it was accidental.'
"What is incidental occurs as a minor consequence of something more important: 'The main advantage of a small car is that it is inexpensive; an incidental advantage is that it is easier to park than a larger car.'"
(Rod Evans, The Artful Nuance: A Refined Guide to Imperfectly Understood Words in the English Language, 2009)

Idiom Alert

The expression accidentally on purpose means as if by accident but actually by intention. If you do something accidentally on purpose, you're only pretending that it occurred by chance. The expression accidentally on purpose is an example of an oxymoron.
Example
"Gently I picked up the fabric scrap. 'I wonder what it was doing behind the mantel. Maybe her mother had propped it up there to display, and then it accidentally fell behind it and was forgotten.'
"'Or maybe,' Jack added, 'her mother accidentally on purpose lost it.'
"I finally looked at him. 'What do you mean?'
"'When I was a little boy, I had a favorite book that I made my mother read to me at bedtime at least sixteen times each night. It had lots of different characters, so when she read it, she had to make up all those voices. It must have been exhausting for her.' He looked down at the sampler for a moment, smiling to himself. 'Anyway, one day the book just vanished. She helped me look and look for that book, but we never did find it. It was only a couple of years ago that she finally confessed to me that she hid it in the bottom of her cedar chest, where I would never find it because she thought she might go mad if she ever had to read it again.'"
(Karen White, The House on Tradd Street. New American Library, 2008)

Practice

(a) When you travel on business, _____ expenses are items such as local transportation, telephone calls, tips, and laundry.
(b) _____ fires are more likely to start in the kitchen than in any other room in the house.

Answers

(a) When you travel on business, incidental expenses are items such as local transportation, telephone calls, tips, and laundry.
(b) Accidental fires are more likely to start in the kitchen than in any other room in the house.