From BR Bullpen

An Opener is a starting pitcher whose role is to be the first pitcher in a bullpen game. He is not expected to pitch more than one or two innings, after which he will give way either to a succession of relief pitchers all making short outings, or to a long reliever who pitches as deep as possible as he can, akin to what a regular starting pitcher would do. Occasionally, the opener may pitch to a few batters before another pitcher, normally a starter, comes in - a manager may arrange this so that the opposing team's top of the batting order less often faces the pitcher who is normally a starting pitcher.

The opener is by definition not eligible for a win, as he will never complete the required five innings, but he can be charged with a loss. Contrary to a regular starting pitcher, he does not need to pace himself, and can come out throwing at maximum effort.

The term was coined by the Tampa Bay Rays in 2018, when they used bullpen games as a regular feature of their starting rotation and had quite a bit of success with the strategy.

Research shows [1] that there have been many instances throughout baseball history of pitchers starting a game, pitching to just one batter, and then leaving. Often it was due to injury, but sometimes it was connected with strategy. The case of Lefty Schegg in 1912 is a notable one (see his page for details) or Curly Ogden in the 1924 World Series.

In a different context, the opener can refer to the first game of a doubleheader, or can be part of the expression "home opener".