In the game of
go,
aji means the threats that one player will have in a position. It means that later on in the game, a player will have an opportunity to make a threat.
However, just because the threat is there doesn't mean it should be used - if responding to it isn't important enough, the opponent might ignore it (tenuki) and play elsewhere.
There is also something called aji-keshi, which means "erase aji". This is when you play a move which allows your opponent to respond while simultaneously fixing the problem. After you have given him this chance, your old threat won't be a threat anymore.
It is important to decide when to use aji - if you use it too early, the opponent will ignore it and play a more profitable more elsewhere. But if you wait too long, you might give your opponent a chance to play there first and fix the problem.
Also, in japanese, aji means "taste". Here is the kanji for it:
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