The DC-CW's fall 2018 three-night crossover will be titled Elseworlds. That name was revealed in a new promotional image sent out by the network.
In comic books, "Elseworlds" is a term DC has used to specify stories outside the main continuity. In the context of the DC-CW, this would include Supergirl's alternate universe but could also include Black Lightning - and possibly even DC shows on other networks such as Fox's Gotham, DC Universe's Titans, and more.
Of course, this event is also scheduled to feature the debut of Ruby Rose's Batwoman and Gotham - could they possibly be coming in from an alternate universe as well?
The Elseworlds crossover is scheduled to begin December 9 with The Flash, followed by Arrow on December 10 then Supergirl on December 11.