A term used by people who write relationship fanfiction to indicate two people in a romantic relationship
(This also expands into other indications of relationships such as the “&” sign which typically indicates a strictly platonic relationship, or a “/“ which typically indicates a mostly sexual or romantic relationship)
Girl on Tumblr: OMG, did you see the new fanfic that got posted that was Character 1 x Character 2?!?! So cute!