This site and others have answers to the question, and all pretty decent. The answers are lists of what we think, and pretty good lists, but they miss one thing that (I think) should be the first item.

Bonsai is art. That means that a tree is an expression by the artist that others can appreciate, but there is something of the individual in each tree, if we want to call it a bonsai. That is part of what we find non-bonsai about mallsais. Each tree is the same species, bent in the same way, in the same pot, and that is a practice exercise, not art. If I take a tree and make it into a bonsai, it will probably be different from what somebody else would do, at least in details, and that is what it should be.

Comments?

My God, how I love bonsai! They are the only thing I could steal without a twinge of conscience! The word "bonsai" from Japanese is translated differently. But the most common and closest to reality interpretation is "a tree in a bowl." Create a mini plant, which exactly repeats the look of a large tree - a real art. Here are a couple of pictures as an opportunity to see the natural landscapes in miniature. As a college student, I even buy an essay to learn more about this beautiful and amazing plants. Once again, this is a whole art! He is already a little less than a half thousand years old. For bonsai are used ordinary trees, they become small by constant cropping and various other methods. In this case, the ratio of the size of the root system, limited by the volume of the plate, and the ground portion of the bonsai corresponds to the proportions of the adult tree in nature.

A bit ironic with this profile picture but I love the images posted

fikus wrote: My God, how I love bonsai!

Just wondering why you have that Ficus Ginseng as profile picture.

fikus wrote: The word "bonsai" from Japanese is translated differently. But the most common and closest to reality interpretation is "a tree in a bowl."

Allthough it is an interpretation indeed, I'd say: not quite. That would probably be more the definition for a hachi-no-ki.
The interpretation heard from someone of our club, who is quite wellknown in the bonsai world, has beent here since the start, has a rich history and really knows his busines:

"Living in a shallow plain"

But just like yamadori, it doesn't translate so easily. You'd have to describe the concept using phrases.

Create a mini plant, which exactly repeats the look of a large tree

And no... that too I do not really agree with. You cannot replicate a real tree in miniature. It is an impression of a real tree - not just any tree, but an old, mature tree, that shows it has struggled against the elements and survived.

Auk wrote: Just wondering why you have that Ficus Ginseng as profile picture.

Me too.

Ivan Mann wrote: Bonsai is art.

I would not per se make this the most important. Rather, bonsai is horticulture.

I would say, bonsai is the art of growing miniature trees, with the aim to create trees that for al purposes mimic a real life old tree, but small without clear traces of human intervention.

leatherback wrote: ... mimic a real life old tree, but small without clear traces of human intervention.

This is highly controversial.
When people start saying that "wire scars add personality to a bonsai", I get goosebumps.

That statement just is an excuse for screwing up wiring the tree. You might feel a tad sorry for the tree, but that is not the same as personality.

No wiring.
At all.