An immaculate inning occurs when a pitcher strikes out all three batters he faces in one inning, using the minimum possible number of pitches - nine. The strikeouts can either be of the swinging or the caught looking variety, and there can be foul balls involved. The purest form of the feat would be to have nine swings and misses. The name for the feat only came into widespread use in the 21st century. Once exceedingly rare - there were none between 1929 and 1952 - immaculate innings began to be more common starting in the 1990s and the evolution in the usage of relief pitchers that led to most such pitchers being used for one inning only. During that inning, they can throw as hard as possible, knowing they do not need to keep anything in reserve.
Also counted are instances when a reliever enters a game with none out in an inning and gets out of it by striking out the next three batters on nine pitches. The ultimate version of that situation occurred on May 8, 2014, when Brad Boxberger of the Tampa Bay Rays came into a game with none out and the bases loaded and proceeded to get out of the jam by throwing nine consecutive strikes, striking out the side.
As of early 2015, there had been 80 immaculate innings recorded in major league baseball history. The first was thrown by John Clarkson of the Boston Beaneaters on June 4, 1889. In the American League, the first recorded instance was by Rube Waddell of the Philadelphia Athletics on July 1, 1902. As expected, only the hardest throwers in major league history have recorded the feat more than once: Sandy Koufax, Chris Sale and Max Scherzer (three times), and Lefty Grove, Nolan Ryan and Randy Johnson (twice each). The most recent pitcher to do so, Kevin Gausman, who recorded his second on August 18, 2019, was a bit of an outlier in that group. Grove and Sale both accomplished the feat twice in the same season.
On June 15, 2022, the Houston Astros became the first team to throw two immaculate innings in the same game against the Texas Rangers: Luis García achieved the feat in the 2nd inning, and reliever Phil Maton repeated it in the 7th. Even more remarkable, the same three batters were involved in both instances: Nathaniel Lowe, Ezequiel Duran and Brad Miller.
- Jonathan Blitzer: "The Ephemeral Perfection of the Immaculate Inning", The New Yorker, August 12, 2017. 
- Ed Eagle: "Immaculate innings: 3 strikeouts on 9 pitches", mlb.com, August 19, 2018. 
- Brian McTaggart: "2 immaculate innings thrown in 1 game for first time", mlb.com, June 15, 2022.