A shutout, abbreviated SHO or ShO, is a game in which the opposing team does not score a run. A pitcher is credited with a shutout if he pitches a complete game and does not allow a run. The pitcher does not need to win the game to get credit for the shutout, as a shutout is recorded for both teams if a game ends with a score of 0-0 (for example, if it is called because of darkness of rain). Thus, both opposing pitchers may record shutouts during the same game.
When more than one pitcher is used by a team that records a shutout, it is known as a combined shutout. While those are usually listed in the league's season statistics (usually as a separate entry after the alphabetical listing of all pitchers), no statistics are kept as to which pitchers have had a hand in the most combined shutouts in a season or in a career (although it's fairly straightforward to look this up).
With the decrease of complete games in Major League Baseball, shutouts credited to starting pitchers have become relatively rare, and a pitcher can now lead the league with two or three in a season, whereas until the 1980s, there were occasionally pitchers with seasonal shutout totals in the double figures. The vast majority of shutouts pitched in the majors in the 21st century are thus combined shutouts.