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I know what KO is, but I have some concerns if I know what exactly it is.
Is it to forbid the repetiton of the same state of the entire table "just after the next two moves" OR "until the game ends"?
For example: is it forbidden if a state of the table repeats after 6 moves? (I tried, it is possible)
Superko is not a rule applied in practice (much). Ko applies only to states potentially repeating after two plays (that are not 'pass').
Bob McGuigan: The word "ko" appears in some situations where some large scale repetition occurs, such as triple ko, but I agree that by itself ko refers to forbidding capture-recapture.
RobertJasiek: See my remarks at the ApproachKo page. Besides one can speak very generally of restriction fights that include ko fights and pass fights and might be considered to start as early as the game. So there is not just one definition of "ko", really. It all depends on what you want to study or observe. There has been a tendency to introduce more and more specialized terms for all kinds of objects. However, currently there is a great confusion of terms rather than a useful system. Grab as many terms as you can and wait for us researchers to offer more useful generalizations in future. Ko is not an easy study object.
Charles I think there is 'just one definition', unless one starts listening to Robert. In which case you are doomed to wander the earth for ever ...
RobertJasiek: Even if "ko" refers to the shape only and not to the fight or circumstances in that the shape occurs and even if the only ko that is called "ko" is the basic ko, there are at least two definitions: One includes the stones/strings adjacent to the two ko intersections, the other excludes the stones/strings adjacent to the two ko intersections. And please note that both kinds of definitions were floating around already when I started learning go. And please ensure that, e.g., direct kos are not kos because they do not refer to only the shape but also to circumstances in that a shape occurs.
The state of the game is a 4-tuple
(board position, number of black stones taken, number of white stones taken, string)
where string is normally the empty string, but (when a ko is being fought) is a single board intersection which is an illegal play. That defines ko.
RobertJasiek: Nice and new. So we now have ko the shape, ko with ko characteristics (like being direct), and ko with associated prohibitions.