Contributed by Coyote

Nephilim are, in Judeo-Christian? mythology and folklore, the offspring between angels who descended to the earthly realm, and human beings, or else are the offspring of regular people and some sort of "great and powerful people".

They are commonly believed to be what the bible refers to as "Giants in the Earth" although I have heard people dispute that.

I have heard that the word Nephilim, or at least it's root, Nephil, translates either to Giant, or Fallen. This is probably where the idea originates that Nephilim are the children of fallen angels, rather than just humans.

From Dictionary.Com:
(Gen. 6:4; Num. 13:33, R.V.), giants, the Hebrew word left untranslated by the
Revisers, the name of one of the Canaanitish tribes. The Revisers have,
however, translated the Hebrew gibborim, in Gen. 6:4, "mighty men."

From Genesis: Nephilim, Dominance and Liberty
They were so named because they fell (naphal) and caused the world to fall <2>. From the Hebrew 'naphal' (to fall) which can mean a fall, a deterioration, in the moral sense.

In the Hebrew language (nephilim) has the meaning of 'giants' {10}. It can refer to mental rather than to physical giants.