I don't get what "kingdom" represents.

Kingdom Hearts is the heart of all worlds. Therefore, Kingdom is needed in there because a kingdom is the "capitol" or "nation" of all cities, villages and towns. Kingdom Hearts is the "Capitol" or "nation" of Traverse Town's heart, Disney's Castle heart, Disney Island's heart, and even Radiant Garden's heart.

 

While this answers seem good, they are all sorta wrong. Let me start by saying, There is no real reason, well that we know of yet why they decided to call it "Kingdom Hearts". Yet if you look at the definition of kingdom; (A realm associated with or regarded as being under the control of a particular person or thing), It comes a little more clear. In each game, when the REALM of Kingdom Hearts was open, someone (the antagonists) tried to control it. Let me go into further detail....

I sorta touched on in above, With the defenition Kingdom Hearts is a realm, that someone wishes control, yet it contains the Heart of something particular.

This is correct, I

Wrong, Kingdom Hearts can be the heart of anything, as we have seen in the game. Pluto can open Kingdom hearts and call it the Heart of all animals, but thats the kingdom hearts he would want to see? get it?

Slightly correct, There are three ways that we know, but there could be more. In the next installment of Kingdom Hearts, well Kingdom Hearts 3, how do you think Xehanort is going to try to open the door to KH? He can't use the PoH's, i doubt he is going to try to recreate the X-Blade, and I don't think Sora, RIku, King Mickey, will help him open it either. There is going to be a fourth way, for all we know there are a lot of ways to open it, but we have really only seen two, Now let me correct the defintions of Kingdom Hearts

KH1: We never really saw the "real" Kingdom Hearts in the first game, for the door was sealed away by Sora and the King, Yet, we can consider "THAT PART" of Kingdom Hearts, the Heart of all worlds.

KH2: In KH2, the kingdom hearts we saw was the REAL DEAL, ", Xemnas may have "expanded" Kingdom Hearts, but it was the real deal, the DtD was opened (which if you didn't know Kingdom Hearts is located behind the DtD) and we entered it, plus it has been confirmed it was the Real Kingdom Hearts (somewhere). Yet this "PART" of Kingdom Hearts was called the Heart of All Men,?

I guess you can say, the two Kingdom Hearts' I just said are "artificial", because of the way they were opened or brought to be.

KHBBS: Keyblade Legacy said it right, well the BBS report says it better:
I have uncovered the Keyblade's ultimate mystery. You see, besides the three families of Keyblades, there is another "Key Blade." While it may sound the same when spoken, it is notated uniquely: "χ-blade." And make no mistake, while it resembles a normal Keyblade, it is something altogether different. Keyblades are said to be man-made counterparts to Kingdom Hearts. The χ-blade, however, coexists with Kingdom Hearts. It is only forged when two hearts of equal power intersect--one heart of pure darkness, one heart of pure light. At the time of its forging, Kingdom Hearts appears. It must be noted, though, that this Kingdom Hearts is special. Unlike the Kingdoms brought about forcibly and artificially through the collection of hearts, THIS Kingdom Hearts is a perfect and complete union of ALL the worlds' hearts. Surely it was over this that the ancient Keyblade War was fought. If so, the walls that divide the worlds today are of little consequence. With the χ-blade, all their hearts could be instantly reunited--and the Keyblade War, refought.

 

I'm not actually wrong about it. Kingdom Hearts is the heart of all worlds. It is true that it can be the heart of anything but I was thinking on facts that was stated that Kingdom Hearts is the heart of all worlds and on the reality as to what a kingdom can be.

 

Kingdom Hearts is a Kingdom where it's heart is the heart to all worlds.