From BR Bullpen
The term pennant refers to the championship of the National League or American League. In 2008, the Philadelphia Phillies won the National League Pennant and the Tampa Bay Rays won the American League Pennant. The pennant winners compete in the World Series.
The use of pennant referring to a league championship goes back to the days when a league champion was awarded a flag, or cloth pennant, that would fly over their stadium the following season. The league pennant is also referred to as "the flag". Today, teams will raise a banner for winning the pennant, and in many cases, just for making the playoffs.