To make prominent; emphasize.
To be a highlight of.
To mark (important passages of text), e.g. with a fluorescent marker pen or in a wordprocessor, as a means of memory retention or for later reference.
To dye (part of the hair) a different color than the rest.
To seek the attention of (a user) on IRC by mentioning their name in a message, causing that message to appear highlighted on their screen.