transitive verb

to emphasize by, or as if by, drawing a line beneath.
The fact that this accident occurred underscores the need for stricter safety measures.

to draw a line or lines underneath (written text); underline.
The child underscored the title of her paper at the top of the page.

noun

a line or lines drawn under written or printed text for emphasis or to indicate italics; underline.

See the full entry