Under what circumstances is an increase in national output called economic growth?
There are actually 2 types of economic growth. Actual growth is when output increases. Potential growth is when productive potential of the economy increases.

If long run aggregate supply increases without any change to short run AS, then you have potential growth but not actual growth. This might happen with if a new discovery is made, but has not yet been applied in industry. 

If SRAS increase without any change to LRAS, then you have actual growth, but no potential growth. This might happen to an oil-dependent economy if there is a temporary fall in oil prices. 

As long as there is short-term capacity in the economy, increases in aggregate demand will lead to actual growth. Aggregate demand, however, cannot lead to potential growth except by affecting LRAS (e.g. through changes in producer expectation etc). 

Finally, you are correct in that actual growth is shown by moving a point closer to the PPC/ in the North East direction.

See any lecture material:
http://www.tutor2u.net/business/strategy/economy-economic-growth.html