its both. the jewish nation was developed among one group of people, and unlike other religions, we dont go actively looking for converts, or make it easy for people to convert, so it kept certain genes 'in the family' so to speak.
a good example of this is tay sachs. which is a condition only jewish people need to get tested for.
Most definitely a religion, but historically Jewish people have certain traits, but obviously over time that will change especially because not every Jewish person will have these specific traits.
Judaism can be thought of as being simultaneously a religion, a nationality and a culture.
Is Judaism an ethnicity? In short, not any more. Although Judaism arose out of a single ethnicity in the Middle East, there have always been conversions into and out of the religion. Thus, there are those who may have been ethnically part of the original group who are no longer part of Judaism, and those of other ethnic groups who have converted into Judaism.
If you are referring to a nation in the sense of race, Judaism is not a nation. People are free to convert into Judaism; once converted, they are considered the same as if they were born Jewish. This is not true for a race.
