In In the Garden of Beasts: Love, Terror, and an American Family in Hitler's Berlin by Erik Larson, what does the word "Aryan" mean?

The term "Aryan" was widely used--and abused--in the 20th century. Most notably, it was used by the Nazis to refer to what they thought was a master race of Nordic Europeans. The actual word "Aryan" comes from the Sanskrit word "arya," which means "noble" and was used by the Sanskrit-speaking people who invaded India around 1800 BCE.

In the late 18th century, European linguists discovered that the Sanskrit language was related to European languages such as Greek and Latin. This discovery caused them to speculate that there was a primordial people that originated somewhere around the Caucasus Mountains that migrated eastward and westward and were the ancestors of both Europeans and eastern people, such as the Iranians and Indians. These people they called "Aryans," and they termed the languages "Aryan" languages. They also called them Indo-Europeans. 

In the late 19th and early 20th century, respectable scholarship about the Indo-European language family fed into some irresponsible theorizing about race. While there is an Indo-European language family, it is not clear whether there is an Indo-European people in racial terms. However, European, especially German, racialists became convinced that there was a so-called Aryan people and that it was a white, northern European people. They thought Aryans were the greatest creator of culture in the history of humanity.

These theories, among other pseudo-scientific theories, were adopted by the Nazis and fueled their propaganda, which asserted that Germans were the master race and that it was appropriate to dominate all others. Finally, under the Nazi regime, the term "Aryan" became the standard term for racially acceptable people. The category excluded Jews, who were believed to be Semitic, and Slavs, though they spoke an Indo-European language. Jews were thought to be unable to exist within German society and a destabilizing influence on that society. To implement their idea of creating a master race, the Nazis instituted a policy of extermination of Jews and other people, including people with disabilities and mental illness.

While the Dodd family, the subject of In the Garden of Beasts, was in Berlin, the Nuremberg Laws were put into effect in Nazi Germany. These laws, announced in 1935, put Germans' ideas about the master race into effect. The laws prohibited Jews from marrying or having sexual relationships with people with German ancestry. They also took away Jews' citizenship in Germany. In the early 1940s, the Nazis began to kill Jews on a mass scale, and they slaughtered 6 million Jews, according to historians, before their brutality was over.