underline

Concise Oxford English Dictionary © 2008 Oxford University Press:
underline/ʌndəˈlʌɪn/

▶verb

• 1 draw a line under (a word or phrase) to give emphasis or indicate special type.

• 2 emphasize (something).

▶noun

• 1 a line drawn under a word or phrase.

• 2 the line of the lower part of an animal's body.