
Anything that does not bring me closer to God is a sin. 
(If you think this is too strict, just consider how ambiguous it is.)

This implies that staying the same is a sin. A Christian should
never be satisfied. It does not imply that
having fun is a sin. It does not imply that sleeping is a sin.
It does imply that I sin every day.

A perhaps simpler definition:
Anything that is counter to the two Great Commandments: 
love God, love your neighbor, is a sin.
Anything I do that is not from love is a sin.

The same action can be a sin sometimes and not a sin sometimes.

I could yell at my kids as discipline, all the time loving them,
considering only to teach them proper behavior, or I could yell at my
kids out of anger or selfishness.

I could post an excellent article because I am interested in sharing
my opinions and getting feedback and learning, or I could post an
article because I want everyone to realize how wise I am.