Since I'm a Christian, a sin means that we have done something bad that is against God's rules or commandments. It is also said that God will always forgive your sins everytime you pray to him or simply go to a confession.
I have been wondering lately what do a sin means to other religions and to atheists?
As an atheist, let me tell you what I believe.
There are universal wrongdoings, such as murder, incest, or stealing which we all consider a sin. But there are grey areas. Such as premarital sex or profanity. These have to be assessed by each individual person to find what they find or not find appropriate. Basically you should know the universal sins and avoid them no matter what, but in regards to the others, if it doesn't feel right then don't do it. If it does feel right and it isn't hurting anyone else then be my guest.