What is good acting?

In the past week I have been fortunate to go to two productions which both got standing ovations. It made me wonder what the difference is between good and great acting. For me it is a case of I know it when I see it, but I have no idea what I am looking for. I suppose it is the ability of the actors to transport the audience into the story and keep us spellbound throughout the play. To create a new believable reality and observe life from another perspective. I would be interested to hear from people who are more knowledgable about this how they define great acting and what it takes to achieve greatness.