Definition: Replace
To place a ball by setting it down by hand and letting it go, with the intent for it to be in play.If the player sets a ball down without intending it to be in play, the ball has not been replaced and is not in play (see Rule 14.4).
Whenever a Rule requires a ball to be replaced, the Rule identifies a specific spot where the ball must be replaced.
See Rules 25.3d and 25.4e (for players who are amputees or use an assistive mobility device, the definition is modified to allow replacing the ball by using equipment).