This is how a brother(named Abu Bakr) who reverted to Islam defined it:
Of reversion generally he stated: “I use the word revert because, I mean, everyone uses convert because that is just a Western ideal. You know (in the Western context) that you have converted to something. But when you look into Islam I guess you realise that the term Muslim just means it comes from Islam, Right? It is a statement of what you are. So Islam means to submit your will to God; to do what God asks you to do; whereas a Muslim is someone who is doing it. So if you are not doing what God has asked you, you are not a Muslim. So, in effect, Muslim is not a noun, it’s a doing word (a verb). Therefore, my tree in the backyard is doing what God asked it. The tree is Muslim to me, you know! It’s submitting its will to God. Everything has energy. So when you’re born, you’re Muslim, you know! ‘I’m hungry’ - you cry - that’s Islamic you know,” he says laughing. “You are acting in accordance with your nature, and it’s only when you get a little bit older, and people teach you stuff, that you go away from Islam because you take on someone else’s ideals. Then when you want to come back, you revert, because you were already Muslim when you were born.”