Niken
Niken = ni ken
means 2 ken
which stands for two intersections on a line (not diagonally or slanted!; this would be hazama tobi and ogeima resp.) on the Go board.
It is thus a measure for the distance between 2 stones,
which can be either of the same colour (niken tobi), or -
in the case of corner approaches, of different colour (e.g. ogeima kakari).