Definition: An "open" golf tournament is one in which participation is not limited only to golfers who have been invited or only to those who have gained entry through some pre-set list of qualifications.
An "open" is distiguished from an "invitational" thusly: The Podunk Golf Association Invitational would only be open to members of the Podunk Golf Association; however, the Podunk Golf Association Open would be open to any golfer in the community who wanted to participate.
So on a local level, any golfer could sign up to play in an "open." On a national level, the U.S. Open is a true "open" because much of the field is made up of players who have advanced through local and regional qualifying events.
