First division is a term that has had various meanings, at various times, in the sport of baseball, but originally referred to the rankings within a league. Teams which ranked in the top 50% of a league were said to have finished in the "first division", while the rest were said to be in the "second division".
In the major leagues before 1969, players on teams finishing in the first division received monetary shares from the Players' Pool, which was funded by gate receipts from that year's World Series; specifically, prior to the aforementioned expansion, the team winning the World Series received 42% of this money, the team losing the World Series got 28%, with the second-, third- and fourth-place teams in each league being awarded 7½%, 5% and 2½% respectively. Concomitant with the expansion, 1% was taken from both of the World Series teams and given to each of the teams finishing in fifth place.
The New York Yankees finished in the first division for a record 39 consecutive years, from 1926 through 1964, all inclusive; by contrast, the Chicago Cubs once finished in the second division for 20 years in a row — between 1947 and 1966.
With the realignment of both leagues into two geographically based divisions in 1969, the terms "first division" and "second division" quickly fell into disuse (largely because the word "division" had acquired a completely different meaning), although teams finishing in the top half of the standings — now the first three teams in each of the new divisions — continued to share in the Players' Pool. The distribution of the Players' Pool money was also changed at this time, with the World Series winner henceforth getting 36% of the pool, its loser 27%, the losers of the League Championship Series 12½% each, the second-place teams in the divisions 2 3/8% each, and the third-place teams 5/8% each. In 1991 these shares were revised to 36%, 24%, 12%, 3% and 1% respectively, and when both major leagues realigned again in 1995 (each league now consisting of three divisions, the winners thereof plus one wild card team in each league reaching the postseason), the 3% shares went to teams losing in the Division Series and the 1% shares went to the teams that finished second in their division but did not earn a wild card.
The terms "first division" and "second division" are rarely if ever used in baseball today, except in a historical context — and even their original meanings have become obsolete since the proportion of Major League Baseball teams sharing in the Players' Pool has fallen significantly below one-half because the number of teams receiving shares has not risen in concert with the increase in the number of teams as the result of expansion: In 1977 the American League expanded to 14 teams, but the number of AL teams qualifying for postseason money remained at six — three in each division, even though the divisions now had seven teams in them. The National League followed suit in 1993, and the addition of the Arizona Diamondbacks and Tampa Bay Devil Rays in 1998 decreased the percentage of major-league teams being awarded Players' Pool shares to two-fifths (12 out of 30).
Originally, the Players' Pool was funded by a portion of the gate receipts from the first four games of the World Series; as baseball added more rounds of playoffs, the receipts from the first four games of best-of-seven series, and the first three of best-of-five series, were included as well, so that players on teams qualifying for shares of the pool receive several times more money today — even adjusting for inflation — than their counterparts did many decades ago.