The description for xss reads:
Cross-Site Scripting attacks are a type of injection problem, in which malicious scripts are injected into the otherwise benign and trusted web sites. Cross-site scripting (XSS) attacks occur when an attacker ...
and that of cross-site reads:
Cross-site scripting (XSS) is a type of computer security vulnerability typically found in Web applications.
UPDATE: The answer below mentions that cross-site often refers to legitimate cross-site functionality. I'm not quite sure. The questions cited as examples appear to be incorrectly tagged as cross-site. Those seem more appropriate to be tagged cross-domain (aka cross-origin).