Ikken tobi is a
Japanese term from the game of
Go (
Wei Qi/
Badouk). It refers to a specific kind of move. In
English, the move it best described by the
phrase "
one-space jump". If you play a stone that is in the same
row or
column as a stone you played earlier, with exactly one empty space between them. In the diagram below, if you have a stone at the spot marked o, a play at any of the spots marked * would be ikken tobi.
.......
...*...
.......
.*.o.*.
.......
...*...
.......
Note that the intervening space must be empty for the move to be considered ikken tobi. If there was a friendly stone in the space, you would have a three-stone wall, and if your opponent had a stone there, the move would be considered a clamp.
There is a Go proverb that says "Ikken tobi is never wrong." Obviously, this is an exaggeration; there is no one move that is always correct, or Go would be an easy game, rather than what is (arguably) the most challenging in existence. However, there is some truth to the proverb. What it actually means is that ikken tobi is a well-balanced move, and appropriate in a wide variety of situations. It is flexible, because either stone may be sacrificed individually. It allows a reasonable amount of development and radiates a decent amount of influence in the direction perpendicular to the jump. At the same time, it is quite strong. The stones are not quite connected, since under some circumstances the opponent might be able to cut them apart by playing in the intervening space, but not without considerable difficulty and associated costs.
This move is perhaps most commonly seen when a weak group comes under attack and is "running" for safety, in order to connect to a stronger friendly group or out into the middle of the goban (Go board). There is another Go proverb that says "Jump once, then make eyes." That is, when making an invasion into an opponent's territory, a good strategy is often to make a single ikken tobi jump towards the centre, then go back and try to make two eyes to live on the spot (see Go node for an explanation of eyes, and why they are necessary for life).
It is also seen as a means of establishing a secure connection between two groups, as shown in the diagram below (I put some x's in to indicate enemy stones or immediate connection would be unneccessary):
.........
.....x...
.o.o.Obc.
.xo...ao.
.xo..xxo.
x.x..xoo.
The played stone, marked with a capital O, is ikken tobi from the top-right stone of the left group, and keima (like a knight's move in chess) from the top stone of the right group. The two groups are now pretty much connected, and much stronger than they would be separately. The opponent might be able to cut by playing X at a, O at B, X at C, but would have to be very strong in the vicinity for this to work.