Not ***the*** name, but ***a*** name: "contradictive assumption". 

Google [knows about 80 some odd uses](https://www.google.com/search?q=%22contradictive+assumption%22+%22proof%22&oq=%22contradictive+assumption%22+%22proof%22) of this in a mathematical context. I like it because the word "contradictive" has a dictionary definition that is more-or-less suited for the job, and the word immediately invokes the method of proof we are using.