The short answer to the definition of human nature is it is the sum of qualities and traits shared by all humans. From a Christian perspective the definition of human nature centers around man's free will (choice) and his relationship to God. 

Our human nature originated with God making man in His image to include giving mankind the freedom to choose [(Genesis 1:27, "So God created man in his own image, in the image of God created he him; male and female created he them.")(Romans 6:16, "Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?")].
 
Free will that is part of our human nature led to Adam's original sin and his sin nature being passed down through the generations of man (Genesis 3:6, "And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat").
 
Thanks to Jesus Christ our human nature does not need to stay under a curse and corrupted by sin with the ultimate result of death [(John 3:16, "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life") (Jeremiah 31:29, "In those days they shall say no more, The fathers have eaten a sour grape, and the children's teeth are set on edge")]. By our faith in Jesus as our Savior and following the Holy Spirit in us, our human nature can be set aside for God's plan and can stay in relationship with God.

A key part of a definition of human nature is defining our sin nature that is in everyone of us. Sin nature is basically a nature that is disobedient to God. We were all born with a sin nature that can be traced back through our ancestors all the way back to Adam and Eve. In the Garden of Eden, God told Adam not to eat of the tree of the knowledge of good and evil (Genesis 2:17). Soon after that Adam and Eve disobeyed God and ate of the tree. This disobedient act toward God was the original sin. This tendency for us to disobey God, our sin nature, continues today in that it was passed down from our forefathers (Romans 5:12, "Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:"). For more commentary on sin, lust, death, separation from God, and pride see links below.

ROMANS 5 BIBLE SCRIPTURE Romans 5:12, "Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:"

God gave us free will; He gave us choice. Free will is a key aspect of the definition of human nature. We were made in His image and God gave us free will in order for us to freely love Him and worship Him. Worshiping God is our purpose and God gave us free will in order for us to fulfill our purpose. The problem with free will is that we can make the choice of being obedient or disobedient to God. Free will results in blessings or curses for us ( Deuteronomy 30:19, "I call heaven and earth to record this day against you, that I have set before you life and death, blessing and cursing: therefore choose life, that both thou and thy seed may live:"). Free will is an inherit part of our human nature. We are not just driven by our instincts or our environment. We are not puppets of God. We choose to be either servants of God or servants of sin and disobedience (Romans 6:16, "Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?"). For more commentary on free will and conscience see links below.

ROMANS 6 BIBLE SCRIPTURE Romans 6:16, "Know ye not, that to whom ye yield yourselves servants to obey, his servants ye are to whom ye obey; whether of sin unto death, or of obedience unto righteousness?"

From a Christian perspective the definition of human nature cannot be explained adequately without explaining the human nature that is in right relationship with God. Our human nature may have the tendency to follow our sin nature, but that is not why God made us. God made us to worship Him. Jesus Christ gives us that opportunity to repent of our sin nature and be in right relationship with God (Romans 8:12-14, "Therefore, brethren, we are debtors, not to the flesh, to live after the flesh. For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. For as many as are led by the Spirit of God, they are the sons of God"). Our human nature does not need to end in corruption and death that sin brings to our lives. If we choose to follow the spirit of God, we can have eternal life and not live in sin (Galatians 6:8, "For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting"). For more commentary on Holy Spirit and Godly relationships see ...

GALATIANS 6 BIBLE SCRIPTURE Galatians 6:8, "For he that soweth to his flesh shall of the flesh reap corruption; but he that soweth to the Spirit shall of the Spirit reap life everlasting."

As God created us, the definition of human nature can't be fully defined without defining our relationship with God. Our human nature at it's best is set aside for God. This is where we put God first. This is where we are willing vessels that purify ourselves to be set aside for God's purpose (1 John 3:3, "And every man that hath this hope in him purifieth himself, even as he is pure"). We progressively become sanctified in our relationship with God. We become partakers of God's divine nature ( 2 Peter 1:4-7, "Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust. And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; And to knowledge temperance; and to temperance patience; and to patience godliness; And to godliness brotherly kindness; and to brotherly kindness charity"). As we purify ourselves in our worship of God we become increasingly sanctified taking on His divine nature of love and goodness. This is the true purpose of our human nature. For more commentary on sanctification and being holy see ...

2 PETER BIBLE SCRIPTURE 2 Peter 1:6 "And to knowledge temperance; and to temperance patience; and to patience godliness;"