The problem is that 'Art' is something that is in no way defined. I study Literature at University, and part of our first year course is 'What is Literature', which touches on the same ideas as 'What is Art'. No-one has of yet come up with satisfactory definitions of this, so attempting to use something as basic as a dictionary, when whole books have been written on the subject is fruitless.
In reality what we determine as 'Art' tends more to mean what we choose, for whatever reason, to say is somehow 'better' than other similar examples of the medium (i.e. painting, sculpture, novels, poetry, films, photography, + even games). The difficulty is in trying to define the reasons some creations are better than others. Mostly these are the value-judgements of an individual, and cannot be made an objective definition. That's not to say that individual value-judgements aren't important, it's just impossible to make a solid, precise or complete definition of them. If we want to attempt to define 'art' despite this, then we have to change what we mean when we say 'art'.
I personally like the idea that Art is the attempt to create "The perfect expression of an idea" - by expression I mean the way the mechanics of the medium are used (in this case a computer game) and by idea I mean almost any aim or purpose, including just to entertain. So, for example, you might say Newton's theory of Gravity was a great idea, and you might say that a beautiful drawing is an incredible piece of craftsmanship, but it is only when the craftsmanship and the idea work together to make both the idea and the expression greater than they could be individually that it becomes Art.
My favourite example of this in gaming is probably Final Fantasy 7, where I thought the ideas behind the characters were made far more powerful because I was playing AS them, and Portal, where the sense of being alone and controlled was far greater because I actually was playing in a controlled environment, alone with Glados: SPOILERS AHEAD!!!
I was actually betrayed and shocked at the end, when it felt like it was actually me being betrayed, and felt genuinely guilty when it was actually MY actions that finally killed her "This isn't brave, it's murder... The difference between us is that I can feel pain. You don't even care, do you? Did you hear me? I said you don't care. Are you listening?"
That is True Art IMO. But as I said, it's only an opinion, and even if it's a particularly good one, it can't possibly be objective fact.
Sorry for length of post