Definition - Provisional Ball

Another ball played in case the ball just played by the player may be:

Rule 18.3a. When Provisional Ball Is Allowed

If a ball might be lost outside a penalty area or be out of bounds, to save time the player may play another ball provisionally under penalty of stroke and distance (see Rule 14.6).

For a ball that might be lost, this applies:

But if the player is aware that the only possible place the original ball could be lost is in a penalty area, a provisional ball is not allowed and a ball played from where the previous stroke was made becomes the player’s ball in play under penalty of stroke and distance (see Rule 18.1).

Official Interpretation 18.3a/1 - When Player May Play Provisional Ball

When a player is deciding whether he or she is allowed to play a provisional ball, only the information that is known by the player at that time is considered. Examples where a provisional ball may be played include when: