Most traditions agree that the original sin was disobedience to God. God specifically told them not to eat of the tree in the middle of the garden.
Sin is described in the Bible as transgression of the law of God (1 John 3:4) and rebellion against God (Deuteronomy 9:7; Joshua 1:18).
Eating of the fruit, in direct contradiction to what He told them to do is the definition of rebellion. They chose to do what they wanted instead of what God had commanded.