A parenthesis is one of a pair of curved marks ( ) used in writing to set off information that's helpful but not essential to the main point. The information inside parentheses adds extra detail, clarification, or a side comment without interrupting the flow of the sentence.

For example, you might write: “My older brother (who just turned thirteen) finally got his own phone.” The part in parentheses gives you extra information about the brother, but the sentence still makes sense without it: “My older brother finally got his own phone.”

Writers use parentheses when they want to include something interesting or useful without making it the focus. A history book might say: “The Declaration of Independence (signed in 1776) established America's independence from Britain.” Scientists use parentheses to add technical details: “The experiment used saltwater (a solution of 3.5% sodium chloride).”

The word parenthesis refers to a single curved mark, but since they almost always come in pairs, people often say parentheses (pronounced puh-REN-thuh-seez) to refer to both marks together. When you're writing, think of parentheses as a way to whisper extra information to your reader without making it the main point.