Wesley's Mother's Definition of Sin

"Take this rule: whatever weakens your reason, impairs the tenderness of your conscience, obscures your sense of God, or takes off your relish of spiritual things; in short, whatever increases the strength and authority of your body over your mind, that thing is sin to you, however innocent it may be in itself." -- Susanna Wesley (Letter, June 8, 1725)


The best definition I ever heard, and I don't recall it's author, is this:

However else it manifests itself, sin is ultimately self-centeredness