Anunaki is often explained as -An una Ki-; "Heaven to Earth" and translated with "those that descended from heaven to earth" (Sitchin).

To the establishment however, dAnunaki ( the silent d stands for Dingir; god or godly)  is a collective name for the gods of heaven and earth, and in other contexts only for the gods of the Netherworld, the empire of the death (in particular beginning in the second half of the second millennium). It is seen as a loan word (plural only) from Sumerian a.nun.(n)a(k) 'seed/descendants of the (-ak) monarch (nun) referring to the offspring of the King of Heaven An/Anu. This seems to be consistent with the ancient texts.

The seven great Anunnaki

caused the Igigi to bear the work.

Anu their father was king.

Atrahasis myth.

 

The Igigi also are called great gods, but only when referring to the great Anunaki.  In general the terms Igigi, or Anunaki stand for the gods in totallity and the Great Anunaki for their leaders. Much like Angels and Archangels. As so often in cases like this, both theories probably apply. Meaning that there probably were astral gods as well as more physical gods indicated with the nouns Igigi or Anunaki.

Sometimes a differentiation is made in the indication of the totality of the gods, the dIgigi and the dAnunaki. The word Igigi is spelled; I-gì-gì; in which í = ia (the word for five in Sumerian) also stands for '5' and twice gì = dish stand for either '2' or '2x60', so as a number it could mean either 5+1+1=7, the number of Great Gods, or 5x2x60 = 600 which is the total number of gods in some traditions.

ina amê u er Setim 600 (gi-u) utêib

litt.: '(and so) he [Marduk] caused 600 (deities) to sit in heaven and earth'

This would be consistent with two hundred fallen angels descending to earth, and One third of the stars of heaven being swept by the Dragons tail.

And they were two hundred in total and they descended on Ardis which is the top of mount Hermon. They called this mountain Hermon because they swore an oath there and bound each other by a curse. 1Enoch 6;6

Behold, a great red dragon, with seven heads and ten horns, and seven diadems upon his heads. His tail swept down a third of the stars of heaven, and cast them to the earth.  Re 12;3

One third of six hundred is two hundred Anunaki or Igigi, or Grigori, that descended to Earth and showed great interest in earthly females;

These are the Grigori, who with their prince Satanail rejected the Lord of light, and after them are those who are held in great darkness on the second heaven, and three of them went down on to earth from the Lord’s throne, to the place Hermon, and broke through their vows on the shoulder of the hill Hermon and saw the daughters of men how good they are, and took to themselves wives, and befouled the earth with their deeds, who in all times of their age made lawlessness and mixing, and giants are born and marvelous big men and great enmity. 2Enoch 18;3

So it would follow, that they descended in physical form.

The Anuna, the great gods, (Anuna-Ki, gods of the earth; earth-gods, chthonic gods) have taken up dwellings in your midst, and consume their food in your giguna shrines with their single trees. Enki and the world order 209

and mixed their races with humans.... 

When men began to multiply on the face of the ground, and daughters were born to them, the sons of God saw that the daughters of men were fair; and they took to wife such of them as they chose. Then the LORD said, "My spirit shall not abide in man for ever, for he is flesh, but his days shall be a hundred and twenty years."  The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. These were the mighty men that were of old, the men of renown. Gen 6;1 RSV

 

The Great Anunaki could be the bundled souls of races living in Space; in heaven, or the gods they worshipped. The Anunaki/Igigi/Grigori could indeed be some of these evolved races of "Aliens" that in time have come to existence throughout the Universe before we did and planned to live on Earth. They were the Nephilim (fallen ones) that descended on Earth just before the Flood wiped them out, after which their souls would survive and hount the human race as evil spirits.

Now the giants, (Also called Nephilim, after their fathers.) who have been born of spirit and of flesh, shall be called evil spirits upon earth, and on earth shall be their habitation. And the evil spirits came out of their flesh, because they were created from the supermondane; from the holy Watchers was their beginning and primary foundation. Evil spirits shall they be upon earth, and the spirits of the wicked shall they be called. The habitation of the spirits of heaven shall be in heaven; but upon earth shall be the habitation of terrestrial spirits, who are born on Earth. 1Enoch 15;8    

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