Sin is anything that is currently preventing you from returning to God's presence.
Jesu Christ said this to the Church of Jesus Christ of Latter-day Saints:
D&C 38: 1-9:
1 THUS saith the Lord your God, even Jesus Christ, the Great I AM, Alpha and Omega, the beginning and the end, the same which looked upon the wide expanse of eternity, and all the seraphic hosts of heaven, before the world was made;
2 The same which knoweth all things, for all things are present before mine eyes;
3 I am the same which spake, and the world was made, and all things came by me.
4 I am the same which have taken the Zion of Enoch into mine own bosom; and verily, I say, even as many as have believed in my name, for I am Christ, and in mine own name, by the virtue of the blood which I have spilt, have I pleaded before the Father for them.
5 But behold, the residue of the wicked have I kept in chains of darkness until the judgment of the great day, which shall come at the end of the earth;
6 And even so will I cause the wicked to be kept, that will not hear my voice but harden their hearts, and wo, wo, wo, is their doom.
7 But behold, verily, verily, I say unto you that mine eyes are upon you. I am in your midst and ye cannot see me;
8 But the day soon cometh that ye shall see me, and know that I am; for the veil of darkness shall soon be rent, and he that is not purified shall not abide the day.
9 Wherefore, gird up your loins and be prepared. Behold, the kingdom is yours, and the enemy shall not overcome.