How do you spell “parenthesis”?

Hint: A word or phrase inserted into a passage that is grammatically complete without it as a clarification or afterthought, usually marked off by brackets, dashes, or commas in prose. An interlude or a period.

Complete answer:
i) Either of a pair of curved marks that look like this is a parenthesis: () and both marks are parentheses.
ii) In parentheses, a symbol, number, expression, phrase, or clause illustrates, supplements, or comments on something in the sentence. Material in parentheses may be omitted from a sentence without altering the overall importance or grammatical integrity of that sentence.
iii) The three punctuation marks that signify an interruption in the flow of a sentence are parentheses, long dashes, and commas.
iv) To form a separate sentence, parentheses may be used, as here: I wished my friend would come. (At the last minute, he canceled.) But this could also have been done as: I wished my friend would come (he canceled at the last minute). Note the location of the time; the period goes after the closing parenthesis if parentheses end a sentence.

Note: A parenthesis is a tall, curvy punctuation mark used to set off content that, like an afterthought or an aside, is not central to the main subject (or a funny joke).
Example: note that the use of this is in this sentence: my friend (and her brother) are coming today. My friend is the focus. Parentheses are never part of the subject, despite appearances. Delete them, and we'd have two subjects that would need the verb to come, my friend and her brother. The use of parentheses is an indicator that the author was more concerned with the friend than about the brother.