Circling back to your question on definition, sin is “an offense against God as well as a fault against reason, truth, and right conscience. Sin is a deliberate thought, word, deed, or omission contrary to the eternal law of God”. 

Essentially, anything that turns one away from God. Yet we don’t need divine revelation to have a sense of what sin is. God created humanity to be able to naturally discern good and evil through use of reason and right conscience. However, we can and do often err so divine revelation by His grace helps us get closer to living a good, loving life than we would be able to otherwise.

Definitions are incredibly important. I encourage you to ask yourself what the definitions of “good”, “evil”, “truth”,“happiness”, “love”,  and “freedom”. At least for me, the more I dug the more I realized the beauty of faith and Church teaching.