Entity or attribute if only one field beside key and attribute for other entity? Let's say I have an entity person which has an attribute Email. But the person can have more than one Email address. So I have to put this attribute into a separate table. 

But what I do not understand in this situation: Is Email now an entity or an attribute? I would say entity, because it has its own table. But I would say attribute, because this is the only function of it.
Its has become its own Entity. You now have a 1:N relationship between Person and Email.