Joseki is a
Japanese word that refers to certain sequences of moves by both players in the
opening of a game of
Go (
Wei Qi/
Badouk). This is sort of similar to the idea of a standard
opening in a game of
chess, but must not be confused with another concept in
Go, called
fuseki.
While a fuseki describes the placement of the first few stones across the whole board, joseki are sequences of moves in one area (usually a corner) of the board. They are categorized by the position of (usually) the first two stones in the sequence, and branch out in a tree-like fashion, so that at several stages of the joseki, one player or the other will have a choice of which branch to follow.
Deviating from joseki is generally thought to be bad for the local position of whoever is doing the deviating, and is often just a mistake, due to the player not having memorized that particular joseki (and all branches thereof). However, depending on the whole board situation, it might sometimes be adviseable.
There is a Go proverb that "Studying joseki loses two stones strength." This is because after a novice has memorized a particular joseki, they may tend to blindly play the joseki through to the end, ignoring branches that they haven't studied, and not giving any thought to the whole-board situation. Therefore, the "best" play locally may actually give them an inferior result globally, compared to more thoughtful, non-joseki moves played by someone who hasn't memorized any joseki.
The most infamous joseki is known as the Taisha Joseki, aka The Joseki of a Thousand Variations, for obvious reasons. It is so complicated that even pros sometimes make mistakes when an opponent leads them into one of its more obscure branches.