From BR Bullpen

A division is a grouping of teams within the league. The current divisions in Major League Baseball are:

The two leagues were first split into two divisions - an eastern and a western division - as a result of the expansion of 1969 which added four teams, two in each league, for a total of twelve teams per league. The divisions originally consisted of six teams each. A new round of postseason play, the League Championship Series, was added that year, pitting the winners of each division title against each other in a best-of-five series to determine who would represent the league in the World Series. This was such a fundamental change to the game that the time period starting in 1969 in MLB is often called the "divisional era". With the expansion of 1977, the divisions in the American League numbered seven teams each, something which also happened in the National in 1993, following another expansion. In the meantime, the LCS had been expanded to a best-of-seven series in 1985.

In 1994, one additional division was created in each league, as well as another round of postseason play, the Division Series. As the number of teams per league - 14 - was not divisible by three, the divisions were uneven, something which the expansion of 1998 failed to resolve, as it was accompanied by the move of the Milwaukee Brewers from the American League to the National League. Thus, the senior circuit had 16 teams and the junior one 14, forcing both to have divisions with uneven numbers of teams. This imbalance was finally resolved in 2013, when the Houston Astros moved from the NL to the AL, giving both leagues 15 teams, split into three divisions of five teams each.

The team that finishes first in a division at the end of the season is said to have won a division title.

Before there were divisions of this kind, a team in the "Second Division" of the league was one who finished in the lower half of the league's standings. (The opposite term is "First Division", although it was used less frequently.)