I use the definition of one who lacks a belief in a God.
I also a slightly different view of how morals are developed. There are three basic parts to it. Genetics, culture and experience.
We are born with certain personality traits. Some kids are shy, some are out going, some are more aggressive, some more passive, etc...
This affect how in the early years a child interacts with the world and starts to cultivate ideas of right and wrong.
The culture of the parents/extended family, what they see on tv, what they are taught in school. Now a days the games they play. When they start to interact with society, that begins to instill ideas of right and wrong. This would include any religious doctrine.
Finally, through their own personal experience. They see how the world is how they affect it and how it affects them. They start to develop their own ideas and what is right and wrong.
So religion and concepts of God can be a part of a person development of morality, but there is plenty of other factors that'd continue to influence their moral development even without the influence of religion.
It ends up being a very complicated path how any individual develops the set of moral values they hold. Of course we all influence one another as well. I have the morals I have. Can't exactly trace how they all got instilled in me. Can't even really say my morals are better than anyone elses. They just happen to be values of right and wrong I currently accept. And, they are still subject to change, though at my age, probably not by much.