Promoted trend is a Twitter ad offer which allows an advertiser’s hashtag or trend to appear at the top of the Trending Topics list on Twitter.
Promoted trends are marked as "Promoted."
Twitter does not give much details about promoted trends but some have been reported to be charged $120 000 on the US market for 24H. Premium days (Oscars night, Superbowl, election, etc) seems to cost considerably more.
Some examples of promoted trends: