1. A side of the board is a sub-board that is 19 x n, for values of n in the range about 7 to 11 (for a useful concept, on a 19 x 19 board).
See for example side patterns.
2. The area near to and including the edge of the board that lies between adjacent corners. It does not include the corners.
3. For more precise regions, see armpit.
The edge of the board means something different, the vicinity of the first line.