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.
See corner approach.
Here we have a No eye Versus No eye Capturing Race. Black has two outside liberties 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.