Based on a 'real' person, John Titor is the name used on several bulletin boards during 2000 and 2001 by a poster claiming to be a time traveller from the year 2036. In these posts he made numerous predictions (a number of them vague, some quite specific) about events in the near future, starting with events in 2004.
In Steins;Gate, it is a codename used by Amane Suzuha on the internet.
Related anime:Steins;Gate TV, 2011
Related manga:Steins;Gate Manga, 2009