Underscore v. t. : To draw a mark or line under; to underline.

Example:
What song would you use to Underscore the Romeo and Juliet balcony scene?

Taxonomy
character, identifiers, C language, external linkage, model, free encyclopedia, implementation, language, hyphen, language support, underline, compound, tradition, clashes, background music, typewriter carriage, computer applications, asterisks.

Defination from the web:
underscore character was the only method of underlining words. ... When the underscore is used for emphasis in this fashion, it is usually interpreted as indicating that the enclosed text is underlined or ...