Definition:

(1) Either or both of the upright curved lines, ( ), used to mark off explanatory or qualifying remarks in writing. Plural: parentheses. Adjective: parenthetical.

(2) The insertion of some verbal unit that interrupts the normal syntactic flow of the sentence. Parenthetical remarks may also be set off by dashes. 

See Examples and Observations below. Also see:

• Notes on Parentheses (A Brief History of Parentheses and a Guide to Their Use)

• Aside
• Brackets
• Comment Clause
• Concessive
• Discontinuity (Grammar)
• Interrupting Phrase
• Parenthetical Details in Capote's Description
• Punctuation

Etymology:
From the Latin, "to insert beside"

Examples and Observations:

• "My very photogenic mother died in a freak accident (picnic, lightning) when I was three."
  (Vladimir Nabokov, Lolita, 1955)
• "Write a three page essay on the subject 'What I Shall Do With My Life' (with a brief account of its chief events to date and a plan for the future).
  
  "Miss Cutbush loved parentheses."
  (Cid Ricketts Sumner, Sudden Glory, 1951)
• "The English (it must be owned) are rather a foul-mouthed nation."
  (William Hazlitt, "On Criticism")
• "'Black dog' is the mood of bottomless, suicidal despair suffered, most notoriously, by Winston Churchill (himself a kind of bulldog in nappies, a logo for Empire; growling and dribbling, wheezing smoke, swollen veins fired with brandy)."
  (Iain Sinclair, Lights Out for the Territory. Granta Books, 1997))
• "It is now necessary to warn you that your concern for the reader must be pure: you must sympathize with the reader's plight (most readers are in trouble about half the time) but never seek to know the reader's wants. Your whole duty as a writer is to please and satisfy yourself, and the true writer always plays to an audience of one."
  (William Strunk, Jr. and E.B. White, The Elements of Style. Allyn & Bacon, 1995)

• "For the vagabond-voyeur (and for travelers voyeurism is irresistible), nothing is not for notice, nothing is banal, nothing is ordinary: not a rock, not the shoulder of a passer-by, not a teapot."
  (Cynthia Ozick, "The Shock of Teapots")
• "Get your facts first, and then you can distort them as much as you please. (Facts are stubborn, but statistics are more pliable.)"
  (Mark Twain)
• "And remember that life's A Great Balancing Act.
  And will you succeed?
  Yes! You will, indeed!
  (98 and ¾ percent guaranteed)
  KID, YOU'LL MOVE MOUNTAINS!"
  (Dr. Seuss, Oh, The Places You'll Go! 1990)
• "A little gravel alley, too small to be marked with a street sign but known in the neighborhood as Shilling Alley, wound hazardously around our property and on down, past an untidy sequence of back buildings (chicken houses, barns out of plumb, a gunshop, a small lumber mill, a shack where a blind man lived, and the enchanted grotto of a garage whose cement floors had been waxed to the luster of ebony by oil drippings and in whose greasy-black depths a silver drinking fountain spurted the coldest water in the world, silver water so cold it made your front teeth throb) on down to Lancaster Avenue, the main street, where the trolley cars ran."
  (John Updike, "The Dogwood Tree: A Boyhood." Assorted Prose. Alfred A. Knopf, 1965)
• The Rise of Parentheses in the 16th Century
  "Humanist scribes had introduced parentheses to isolate interpolated expressions which were grammatically independent of their immediate contexts, but in the 16th and 17th centuries they were employed (especially in England) much more freely than at any other time. Any expression which might be regarded as parenthetical was enclosed within the two marks."
  (M.B. Parkes, Pause and Effect: An Introduction to the History of Punctuation. Univ. of California Press, 1993)
• Punctuating Parenthetical Remarks
  "If a quoted sentence that originally ended with a period is inserted somewhere in the middle of another sentence, the opening and closing quotation marks are retained, but the period is deleted . . .. If it originally ended with a question mark or an exclamation point, that punctuation is retained and followed by closing quotation marks and a closing parenthesis. If the insertion concludes a sentence, the appropriate end punctuation (e.g., period, question mark, or exclamation point) is placed after the concluding parenthesis."
  (David K Woodroof, Woodroof's Quotations, Commas and Other Things English. iUniverse, 2005)
• Using Dashes to Set Off Parenthetical Remarks
  "In the valley of the jolly--ho-ho-ho!--Green Giant."
  (commercial jingle for Green Giant foods)
  
  "I walk around with a cane now when outdoors--'Stop brandishing!' I hear my wife, Carol, admonishing--which gives me a nice little edge when hailing cabs."
  (Roger Angell, "This Old Man." The New Yorker, February 17, 2014)
  
  "If anyone thinks very intently on a single idea, with concentration and sustained attention, he will become conscious of a slight quiver or creeping feeling--it has been compared to the creeping of an ant--in the pineal gland."
  (Annie Besant, Thought Power. Quest Books, 1952)
• Multiple Parenthetical Remarks
  "And yet, if you watched the news--especially the epileptic seizure that passes for news on cable television (and in certain precincts of the blogosphere)--you'd think that we were facing Armageddon, Sodom, Gomorrah, and the last days of Pompeii all at once."
  (Joe Klein, "The Year of Living Predictably." Time, Jan. 10, 2011)
  
  "At last it is Curly who picks up the plank, rough hewn and smelling of sweet gum, and--feeling the weight and heft and fiber of it--swings it innocently (bending to retrieve the tool, the ball-peen hammer dropped casually on Larry’s toe) and feeling the awful force of the blow as it (the plank) catches Moe upside his head . . .."
  (David Sheffield, winner of the 2004 Faux Faulkner contest)
   
• The Danger of Overworking Parentheses
  "Parenthetical remarks of this sort--which may also be punctuated with dashes--can be a source of interest and variety as well as of necessary information. Moreover, such intrusions loosen the rhythm of a sentence, suggesting more interesting patterns of speech. The effectiveness of parenthetical remarks, however, depends on their scarcity. Using one in every other sentence costs you whatever advantage the device had, and overused parentheses can become an irritating mannerism."
  (Thomas S. Kane, The New Oxford Guide to Writing, Oxford Univ. Press, 1988)
• C.S. Lewis's Parentheses
  "[I]n Calormen, story-telling (whether the stories are true or made up) is a thing you're taught, just as English boys and girls are taught essay-writing. The difference is that people want to hear the stories, whereas I never heard of anyone who wanted to read the essays."
  (C.S. Lewis, The Horse and His Boy, 1954)
  
  "[C.S.] Lewis's prosestyle is faultless. I admired his use of parenthetical statements to the reader, where he would just go talk to you. Suddenly the author would address a private aside to you, the reader. It was just you and him. I'd think, 'Oh, my gosh, that is so cool! I want to do that! When I become an author, I want to be able to do things in parentheses.' I liked the power of putting things in brackets."
  (Neil Gaiman, interviewed by Hank Wagner in Prince of Stories: The Many Worlds of Neil Gaiman. Macmillan, 2008)
• Henry James's Parentheses
  "[Henry James] talked, as he wrote, in long involved sentences with a little murmur--mum-mum-mum--standing for parentheses, and with these rhetorical hooks he seemed to be poking about his mind, fumbling through the whole basket of his conversational vocabulary, to find the exact word, which he used in talking about most ordinary matters. He seemed to create with those parentheses."
  (William Allen White, The Autobiography of William Allen White, 1946)
• Sarah Vowell's Affection for the Parenthesis
  "I have a similar affection for the parenthesis (but I always take most of my parentheses out, so as not to call undue attention to the glaring fact that I cannot think in complete sentences, that I think only in short fragments or long, run-on thought relays that the literati call stream of consciousness but I still like to think of as disdain for the finality of the period)."
  (Sarah Vowell, "Dark Circles." Take the Cannoli: Stories From the New World. Simon & Schuster, 2000)
• The Lighter Side of Parentheses
  "If you can't hear me, it's because I'm in parentheses."
  (Steven Wright)

Pronunciation: pa-REN-thi-sis

Also Known As: round brackets (chiefly British)