What is a "walk-off" hit?

Which league first adopted the designated hitter in 1973? What rule did MLB introduce in 2023 to speed up the game? What is a home run with the bases loaded called? What does it mean when a batter "hits for the cycle"? What does "RBI" stand for? What does "ERA" stand for in pitching statistics? What does a "K" mean on a baseball scorecard? What does a backwards "K" mean on a scorecard?