Novus ordo seclorum is the Latin motto suggested in 1782 by Charles Thomson, the Founding Father chosen by Continental Congress to come up with the final design for the Great Seal of the United States.
On June 20, 1782, Congress approved Thomson's design for both sides of the Great Seal whose official description for the reverse side specifies:
On the base of the pyramid the numerical letters MDCCLXXVI & under neath the following motto. "novus ordo seclorum"
Although Thomson did not provide an exact translation of the motto, he explained its meaning in conjunction with the date 1776 on the base of the unfinished pyramid:
the words under it signify the beginning of the new American Æra
Translating NOVUS ORDO SECLORUM
An accurate translation of Novus Ordo Seclorum is "A New Order of the Ages," but the meaning of this motto is better understood when seen in its original context.
Discover the Source of Novus Ordo Seclorum.
The farsighted founders of the United States looked back into history as well as forward, realizing their actions would have long-lasting consequences for future generations.
In January 1776, Thomas Paine inspired the Colonies with a vision of this new American Era. In Common Sense, he wrote: "The cause of America is in a great measure the cause of all mankind... 'Tis not the concern of a day, a year, or an age; posterity are virtually involved in the contest, and will be more or less affected, even to the end of time, by the proceedings now."
In his June 1783 farewell letter to the Army, General George Washington wrote: "The foundation of our Empire was not laid in the gloomy age of Ignorance and Superstition, but at an Epoch when the rights of mankind were better understood and more clearly defined, than at any former period."
Novus ordo seclorum does not mean "new world order."
President Franklin D. Roosevelt speaks about "A New Order of the Ages."
There's More to This Pyramid Than Meets the Eye
For an in-depth report on why the Great Pyramid was chosen to symbolize America, read "The Great American Pyramid" – researched and written by the author of GreatSeal.com.
The phrase is taken from the fourth Eclogue of Virgil, which contains a passage (lines 5-8) that reads:
The forms saecla, saeclorum etc. were normal alternatives to the more common saecula etc. throughout the history of Latin poetry and prose. The formsaeculorum is impossible in hexameter verse: the ae and o are long, the u short by position. For the medieval exchange between ae, æ and e, see Æ; the word medieval (mediæval) itself is another example.
Medieval Christians read Virgil's poem as a prophecy of the coming of Christ. The Augustan Age, although pre-Christian, was viewed as a golden age preparing the world for the coming of Christ. The great poets of this age were viewed as a source of revelation and light upon the Christian mysteries to come. 
The word seclorum does not mean "secular", as one might assume, but is the genitive (possessive) plural form of the word saeculum, meaning (in this context) generation, century, or age.Saeculum did come to mean "age, world" in late, Christian Latin, and "secular" is derived from it, through secularis. However, the adjective "secularis," meaning "worldly," is not equivalent to the genitive plural "seclorum," meaning "of the ages."
Thus the motto Novus ordo seclorum can be translated as "A new order of the ages." It was proposed by Charles Thomson, the Latin expert who was involved in the design of the Great Seal of the United States, to signify "the beginning of the new American Era" as of the date of the Declaration of Independence.