A word search for Innocent reveals many examples to help us with a definition. The Bible clearly tells us that we are sinners worthy of condemnation (Romans 3:19-23), so innocence as used in the Bible is relative to one's actions in the society, just as it is in normal usage.
The innocent obey the laws and are fair to their neighbors, so when they are preyed upon by the unscrupulous, they are identified as "innocent" because they themselves did not initiate the evil brought upon them:
Deuteronomy 27:25 - Cursed be he that taketh reward to slay an
innocent person. And all the people shall say, Amen.
Also, it seems that they are innocent because the evil done against them violated the Lord's will, and the violence upon them was unjustified by the perpetrators:
2 Kings 21:16 - Moreover Manasseh shed innocent blood very much, till
he had filled Jerusalem from one end to another; beside his sin
wherewith he made Judah to sin, in doing that which was evil in the
sight of the Lord.
Exodus 23:7 - Keep thee far from a false matter; and the innocent and
righteous slay thou not: for I will not justify the wicked.
The innocent also have a strong moral sense, and take action against the wrong, as we see in this parallelism with "upright" and "innocent":
Job 17:8 - Upright men shall be astonied at this, and the innocent
shall stir up himself against the hypocrite.