Preliminary final is an Australian term referring to a game in a knockout tournament or bracket whose winner progresses to the final or championship game (often called the Grand Final). It is equivalent to a "semi-final" in many other sports.
Typically, in finals series that involve preliminary finals, "semi-finals" decide who will go through to the preliminary finals.
For example, in a variation of a finals series where top four teams qualify, the first round will often include a qualifying semi-final (1st v 2nd) and an elimination semi-final (3rd v 4th). The winner of the qualifying semi-final will go through to the grand final, while the loser will go through to the preliminary. In the elimination semi-final, the loser is eliminated, and the winner progresses through to the preliminary final.
Examples of other variations of final series involving preliminary finals can be seen in McIntyre Final Eight System or the AFL Finals Series.
At the top professional level such as the Australian Football League or National Rugby League, preliminary finals are often considered to be better quality games than grand finals. For one thing, commentator Daniel Mueller has made the point that grand finals tend to result in blowouts since one team either chokes under the immense public pressure; or becomes demoralised and effectively 'gives up' after the other team establishes a significant lead. In addition, grand finals are often associated with pageantry, celebrity, bandwagoning, and the fickle big-event fan. These are aspects often looked down on by purists of the game.