To underline means to draw a line beneath a word or phrase to make it stand out. When you underline something in your notes, you're marking it as important or worth remembering. Teachers often ask students to underline key vocabulary words in a reading assignment, or to underline the topic sentence in each paragraph.

Before computers and word processors became common, underlining was the standard way to show emphasis in handwritten or typed documents. Today, we often use italics or bold text instead, but underlining still appears frequently in handwriting and some printed materials.

The word also means to emphasize or highlight something's importance, even without actual lines. When a scientist says her research underscores (a word related to underline) the need for clean water, she's drawing attention to that urgent issue. If a coach tells the team that their last loss underlines the importance of practice, she means it makes that point especially clear and memorable.