D&C 105

"...the Lord called his people ZION, because they were of one heart and one mind, and dwelt in righteousness; and there was no poor among them."  (Pearl of Great Price | Moses 7:18)

As the so called "Zions Camp" was encamped at Fishing River, the Lord explained that -

"...were it not for the transgressions of my people, speaking concerning the church and not individuals, they might have been redeemed even now.
3 But behold, they have not learned to be obedient to the things which I required at their hands, but are full of all manner of evil, and do not impart of their substance, as becometh saints, to the poor and afflicted among them..."  (Doctrine and Covenants | Section 105:2 - 3)

And then He taught the concept of Zion telling them that they -

"...are not united according to the union required by the law of the celestial kingdom;
5 And Zion cannot be built up unless it is by the principles of the law of the celestial kingdom; otherwise I cannot receive her unto myself."  (Doctrine and Covenants | Section 105:4 - 5)

Zion then, in my mind, would be living in a manner and according to the laws of the celestial kingdom on earth.  Therefore, Zion would then be celestial life on earth.

Obviously, this was and still remains very difficult as we as saints are still striving to overcome the world, their selfish inclinations, and to repent and commit to such a life.  The Lord knew that, that is why He then explains to these early saints -

"That they themselves may be prepared, and that my people may be taught more perfectly, and have experience, and know more perfectly concerning their duty, and the things which I require at their hands.
11 And this cannot be brought to pass until mine elders are endowed with power from on high.
12 For behold, I have prepared a great endowment and blessing to be poured out upon them, inasmuch as they are faithful and continue in humility before me."  (Doctrine and Covenants | Section 105:10 - 12)

Though this may be more in reference to the endowment of power received in the Kirtland Temple, I also believe it also has great reference to the endowment given in all temples.  It is therein that we really learn our duties, receive covenants, and endowed with power and knowledge that put us on the course to live according the laws of the celestial kingdom or become one of Zion.