In baseball, the cleanup hitter is the hitter who bats fourth in the lineup. Although the third man up is generally the hitter with the highest batting average, cleanup hitters often have the most power on the team and are typically the team's best power hitter; their job is to "clean up the bases", hence the name.
The theory behind the cleanup hitter concept is that at the beginning of the game, the first three batters will reach base with a single-base hit, walk, or equivalent, thereby loading the bases, and enabling the cleanup hitter a chance to hit a grand slam home run. This, however, is a rare occurrence.
Justin Morneau holds the record of most games started in a season batting cleanup, with 163 in 2008.
Read more about Cleanup Hitter: Other Countries, Notable Cleanup Hitters