The name Nephilim is Hebrew in origin, and means fallen ones. Nephilim is also a plural word, which refers to more than one Naphal. Nephilim, which are occasionally called Nefalim, are creatures who exist within the Hebrew Bible. They first appear in the Old Testament Book of Genesis, and then later on within the Book of Numbers. Outside of the Old Testament, numerous references to them can also be found in noncanonical Jewish literary works. There is a whole host of scholarly debate and myth surrounding these creatures, especially with regard to their Biblical roots and origin. Some theologians believe the Nephilim were the hybrid offspring of a group of fallen angels that bred with human women. Other Biblical scholars disagree with that first source of origin, and state that the Nephilim are actually the descendants of Seth and Cain. To this day the debate regarding who the Nephilim truly are rages on, with no end or solid conclusion in sight.
Abraham ibn Ezra, one of the most famous Jewish writers of the Middle Ages, has concluded that the reason the Nephilim were referred to as the fallen ones, is because anyone who looked upon them would instantly fall to the ground dead with fear. Abraham also suggested a second meaning of their name, which proposed that as the Nephilim were huge giants, that when they fell to the ground in battle, the whole earth shook which caused any human who was standing near them to fall to the ground too, as if a massive earthquake had just happened. This theory obviously goes against the notion that they were called the fallen ones, because they were the descendents of the fallen angels that were cast out of heaven during Lucifer's rebellion against God. The two proposals put forth by Abraham ibn Ezra, have been met with quite a bit of skepticism from certain groups of Jewish and Christian scholars, who mostly disagree with his conclusions.
The Targum Jonathan has claimed that the Nephilim are referred to as the fallen ones, because they are indeed the descendants of fallen angels. The foundation for this theory lies within the Book of Genesis, which seems to parallel statements that were also made within the Epistle of Jude. This scripture states that the undisputable paternity of the Nephilim are angels from heaven, who were thrown out by Michael. It then goes on to state that these fallen angels took human wives, and that they reproduced with them creating hybrid offspring. In the Book of Genesis Chapter 6 verse 4, the scripture implies that the Nephilim have roamed the earth twice, both before and after the great flood. Taking these passages into account, one would conclude that the Nephilim were superhumans, and giants amongst men. Many scholars are absolutely convinced that the giants of Canaan from the Book of Numbers Chapter 13, are in fact Nephilim.
Another theological interpretation of the fallen ones name and meaning, is that the reference means that they were fallen humans, as in men that had fallen from God's grace. There is further scriptural backing that might lead one to translate scripture to mean that they were the sons of nobles, not the sons of heaven, meaning they were not descended from fallen angels, merely high ranking social figures. This conclusion has been linked directly to Seth and Cain, and it has been proposed that they are the true source of the Nephilim. The Biblical academic consensus doesn't seem to favor this theory, as the Book of Genesis lies a pretty solid foundation for a counter conclusion. The reason such vastly different theories regarding the Nephilim have been put forth, is because decoding the separate translations of the Bible leaves a lot of space for errors to develop. It's unlikely the origin of the Nephilim will ever be solved.