Originally Posted by soaringhigh650
But separation is collision avoidance, is it not?
If you're not separate from each other, you must be joined. Therefore you've collided.
In ATC terms the word separation has a rather specific meaning and relates to a number of defined minimum 'distances' that ATC must keep certain aircraft apart. These distances are defined in a number of ways so that they can be applied in different situations, for example as times between aircraft passing a particular point. When using radar it's usually more simple and you might hear references to 5 miles and 1000ft, these being the minimum lateral and vertical distances respectively - in this case the controller must ensure one or the other separation distances exists between two aircraft being separated.
Depending on the class of airspace and the flight rules that the aircraft are flying under, the controller will either be required to separate two aircraft or not. If separation is not required the controller will have some responsibility to provide information to help the pilot(s) to avoid a collision. There is no minimum distance by which a pilot needs to miss the other - this is collision avoidance. The amount of information that ATC must provide varies, again depending on the class of airspace and local practice.