Jihadi John is the nickname given to The Islamic State member who beheaded seven civilians (James Foley, Steven Sotloff, David Haines, Alan Henning, Peter Kassig, Haruna Yukawa, and Kenji Gotō) who were captured by said terrorist group. He is also known as John the Beatle, due to being part of ISIS terrorist cell called The Beatles.
On February 26th, 2015; he was identified as Mohammed Emwazi (August 17th, 1988 - November 12th, 2015). He was killed in a drone strike in Syria the day before the attacks in Paris.
See also: ISIS