M42 refers to the 42nd Mersenne prime 225,964,951-1.
M42 was discovered to be prime on 2005-02-18 by Martin Nowak, using Prime95 written by George Woltman. At time of its discovery, it was the largest known prime number. The number is 7,816,230 decimal digits long. This prime number was the eighth record prime found by the GIMPS project.
- GIMPS Discovers 42nd Mersenne Prime (press release)