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Wiktionary
spring training

n. 1 (context baseball English) A set of practices and exhibition games for each Major League Baseball team, preceding the start of the regular season, which are used by a manager to determine the roster for the regular season. 2 (context noun adjunct baseball English) Of or pertaining to the set of practices and exhibition games preceding the start of the regular season.

WordNet
spring training

n. preseason training during the spring

Wikipedia
Spring training

In Major League Baseball, spring training is a series of practices and exhibition games preceding the start of the regular season. Spring training allows new players to try out for roster and position spots, and gives existing team players practice time prior to competitive play. Spring training has always attracted fan attention, drawing crowds who travel to the warmer climates to enjoy the weather and watch their favorite teams play, and spring training usually coincides with spring break for many US college students.

Spring training typically starts in mid February and continues until just before Opening Day of the regular season, traditionally the first week of April. In some years, teams not scheduled to play on Opening Day will play spring training games that day. Pitchers and catchers report to spring training first because pitchers benefit from a longer training period. A few days later, the position players arrive and team practice begins. Team members normally wear their batting practice uniforms for the duration of spring training and only wear their normal jerseys beginning on Opening Day.