Open golf tournament
The descriptor "open" is used in a number of sports, and especially in golf, to describe a tournament that is "open" — in theory to all — rather than being closed, i.e., one that is restricted to a particular group. Thus, an amateur tournament, e.g., the U.S. Amateur, is not open, because it is, by definition, closed to professional golfers.  The first precise usage of the adjective “open” in golf was in 1861 when the Prestwick Golf Club opened its Medal competition (launched the previous year for professionals only) to amateurs as well; ever since, The Open Championship has been open to all. Any golfer can, in theory, play in The Open either by invitation or by various forms of qualification. In practice, the event is a professional tournament in which a small number of the world’s leading amateurs also play, by invitation or qualification. 

Over time the descriptor "open" has been extended to include the premier national tournaments of many countries (starting with the U.S. Open in 1895) and also (mainly in the United States) to some other tournaments, e.g., the Buick Open.

Principal national "Opens"

PGA Tour
United Kingdom – The Open Championship
United States – U.S. Open
Canada – Canadian Open

PGA European Tour
United Kingdom The Open Championship
Austrian Open
China Open
Hong Kong Open
South African Open
Malaysian Open
Indian Open
Indonesia Open
Turkish Open
Madeira Island Open
Portuguese Open
Asian Open
Spanish Open
Italian Open
Irish Open
Wales Open
French Open
European Open
Scottish Open
Dutch Open
Russian Open

Asian Tour
Bangladesh Open
Brunei Open
Mauritius Open
Korea Open
Myanmar Open
Pakistan Open
Philippine Open
Singapore Open
Thailand Open (golf)

Sunshine Tour
Africa Open
South African Open
Zambia Open
Zimbabwe Open

PGA Tour Latinoamérica
Argentine Open
Colombian Open
Dominican Republic Open
Ecuador Open
Guatemala Open
Mexican Open
Peru Open

LPGA Tour
Women's Australian Open
Women's British Open
Canadian Women's Open
New Zealand Women's Open
Ladies Scottish Open
United States Women's Open Championship

Ladies European Tour
Open de España Femenino
Open de France Dames
Women's Indian Open
Qatar Ladies Open

Others
Australian Open
Japan Open
Kazakhstan Open
Kenya Open
New Zealand Open