What does it mean when a batter "hits for the cycle"?

In what year did MLB adopt the designated hitter in both leagues? 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 is a "walk-off" hit? What does "RBI" stand for? What does "ERA" stand for in pitching statistics? What does a "K" mean on a baseball scorecard?