What are the Nephillim? Most don't understand the concept, and therefore theorize what they were, and whence they came. Common theory would have you believe that they are the offspring of Angels and Women, using the term "sons of God" as their basis for Angels, however what is the reality?
The truth lies within Old and New Testament context. Biblically, who are the "sons of God"?
First, we'll list the scriptures that we are using for validation. Where does it mention the Nephilim in the Bible?
Genesis 6:4 is where most first look, however in the King James Version of the Bible, it writes clearly:
"1And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, 2That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. 3And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. 4There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown." (Hebrew OT - Transliteration - Holy Name KJV)
You'll see that the term Nephilim is not used at all, however it does refer to the sons of God.
The other scripture in the Old Testament that most like to use in regards to the Nephilim is Numbers 13:33, which states (using the King James Bible):