An underline is a more or less horizontal line immediately below a portion of text. Generally, it is used to emphasize keywords or important passages. Not to be confused with "underscore", which is used for the character "_".
Underline in CSS
In CSS, underlining is created by setting the property "text-decoration" to "underline" (other line types, like strikethrough or overline, are also possible). The property "text-decoration-style" defines the appearance of the underline (e.g. solid, dotted, wavy). In CSS3, underline color can be defined by "text-decoration-color" to be different from the text color. For more information, see w3schools.com.
Underline in HTML
HTML has a presentational element <u> for underlined text. However, it was deprecated in HTML 4.01 in favor of CSS; in HTML5 it was redefined for text that should be "stylistically different" (cfr. w3schools.com).
Underline in Unicode
In Unicode, underline can be created by the combining diacritic "combining low line" (U+0332, "◌̲ ").
Underline in ASCII
In plain-text (ASCII) environments, underlining is often indicated by surrounding underscore characters, e.g. "This text is _underlined_."
