What is the breath of life?

Genesis 2:7 (KJV) And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. 

Acts 17:25 (KJV) Neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things; 

Genesis 6:17 (KJV) And, behold, I, even I, do bring a flood of waters upon the earth, to destroy all flesh, wherein is the breath of life, from under heaven; and every thing that is in the earth shall die. 

Job 27:3 (KJV) All the while my breath is in me, and the spirit of God is in my nostrils; 

Job 34:14-15 (KJV) 14 If he set his heart upon man, if he gather unto himself his spirit and his breath; 15 All flesh shall perish together, and man shall turn again unto dust. 

Isaiah 40:7 (KJV) The grass withereth, the flower fadeth: because the spirit of the Lord bloweth upon it: surely the people is grass. 

Isaiah 42:5 (KJV) Thus saith God the Lord, he that created the heavens, and stretched them out; he that spread forth the earth, and that which cometh out of it; he that giveth breath unto the people upon it, and spirit to them that walk therein: 

Isaiah 57:16 (KJV) For I will not contend for ever, neither will I be always wroth: for the spirit should fail before me, and the souls which I have made. 

Job 33:4 (KJV) The spirit of God hath made me, and the breath of the Almighty hath given me life.