Seki refers to a situation smiliar to a "stalemate" in Chess. If a vacant space is surrounded by both white and black stones, in such a way that neither player can play inside without losing the group, that is seki.

Here, White cannot try to kill Black by approaching, without reducing White's liberties to 1, allowing Black to capture all the white stones. Similarly, Black cannot approach the white stones.

Thus neither side can claim the two empty points (4a,4b) and this is a local impasse, giving no score to either side. This is not a true "stalemate" like in Chess, though, the game can continue: it's just a local draw.

[Note that under Chinese rules, the scoring is different. In this situation,

Chinese rules will count two points of White territory.]