Position
A position is the arrangement of black and white stones on the go board. Position is a term used in different contexts, like positional judgement or the restriction for repetition of a position in superko rules.
The term position may also be used to refer to the distribution of stones on a specific region of the board. To avoid ambiguity one may refer to a local position or a whole board position, where appropriate (as in a superko rule).
Formal Definition
Given a grid graph? G = (V, E), the position consists of the color information of all vertices (intersections), where the color can be "black", "white", or "empty".
A position can either be legal or illegal. A position is legal if and only if no strings of stones that have no liberties exist.
Number of positions
On the 19×19 board, there are 361 intersections. Since each of these can be either black, white, or empty, there are 3^361 (about 10^172) possible positions on that board. Not all of them are legal. Research by John Tromp and Gunnar Farnebäck has shown that approximately 1.2% of board positions are legal (no stones without liberties exist on the board), which makes for about 10^170 legal positions. See also: Number of possible go games.