A big move is a move that develops first in open area (opening), condiderably widens a sphere of influence (middle game) or adds a lot of points to existing territory (endgame).
A big area is a large open area, waiting for the first player to claim it.
Big can also be understood in terms of efficiency. If a minimum number of stones is used to effectively claim a certain area, the (prospective) territory can be called big.
- oba or big point