Fore!
The traditional warning shout when a ball might be heading toward someone, always yell it, immediately, if there's any doubt.
More course & etiquette terms
Honor
The right to tee off first on a hole, traditionally earned by having the lowest score on the previous hole.
Ready Golf
An informal approach to pace of play where whoever is ready hits, instead of strictly playing in honor order, common (and encouraged) in casual rounds.
Out of Bounds (OB)
The area outside the legal boundaries of the course, marked with white stakes, hitting a ball there costs stroke and distance.
Penalty Area (Hazard)
Water or other marked areas (yellow or red stakes/lines) where a ball landing inside carries a one-stroke penalty and specific relief options.
Provisional Ball
A backup ball played when the original might be lost or out of bounds, to save time walking back if it is.
Casual Water (Ground Under Repair)
Temporary standing water or a marked damaged area on the course from which a golfer is entitled to free relief.