Each of us may have our own definition of sin, but this one from Susannah Wesley to her preacher son is …really amazing.
….whatever weakens your reasoning,
….what ever impairs your tenderness of conscience,
….whatever obscures your sense of the things of God,
….whatever takes away your desire for spiritual things,
In short ….whatever increases the power and authority of the flesh over the Spirit,
Then, this thing is sin, no matter how innocent it may be in itself.