Approach Move
Approach move is a general term which includes any move which reduces the liberties of an opposing stone or chain of stones. See approach ko.
Approach moves in the opening
See corner approach.
Approach moves in capturing races
Although you can easily count apparent external liberties, sometimes it is necessary to play on more points than just the dame to capture a group.
Here we have an example of a No eye Versus No eye Capturing Race. Black has two outside liberties both marked a. White has one outside liberty at b but Black has an approach move c to make in order to capture White. So we can count c as another outside liberty. There is only one inside liberty: White wins even if Black has sente.