Approach move is a general term which includes any move which reduces the liberties of an opposing stone or chain of stones. They are usually needed to repair weaknesses in the attacking stones. When one or more are required to win a ko, it is called an approach ko.
A corner approach, the first move near a single opposing stone in a corner is usually not called an ‘approach move’, but simply a corner approach or kakari.
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. This is often because of shortage of liberties of the attacking stones.
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 shared liberty, at : White wins even if Black has sente. Black’s problem is the shortage of liberties of his stone .