"...for all have sinned and fall short of the glory of God.." (Rom 3:23, NIV)

Falling short of God's glory - His standard or mark of righteousness and holiness.

Sin is lawlessness (1 John 3:4) or transgression of God's will, either by omitting to do what God's law requires or by doing what it forbids.

The transgression can occur in thought (1 John 3:15), word (Matt 5:22), or deed (Rom 1:32).

Mankind was created without sin, morally upright and inclined to do good (Eccl 7:29).

The teaching of Mencius about "Man's nature is originally good" is not totally without Biblical roots.

But sin entered into mankind when Adam and Eve disobeyed God by eating the forbidden fruit in the Garden of Eden (Gen 3:6).

Being the head of the whole human race, Adam's sin affected all future generations (Rom 5:12-21). His fallen nature was passed on to all his descendants. Out of this, man committed all kinds of sins (Matt 15:19); and no one is free from sin (Rom 3:23).

Source:
Nelson's Illustrated Bible Dictionary