Meaning of アマ in Japanese

This year too there are many regular concerts for amateur musicians being held.

The Amazon River meanders through northern Brazil.

I've ordered a book from Amazon.com.

He explored the Amazon jungle.

The doorman did not permit me to enter the theater.

The point we must clarify here is that the lines between amateur and professional in sport are beginning to disappear.

The Amazon is the second longest river in the world after the Nile.

The Amazon is fed by a large number of tributaries.

There are so-called primitive cultures in the jungles of the Amazon and on the island of New Guinea, and there are so-called advanced cultures in Europe, Asia, and Africa, but the languages of these cultures are all equally advanced and complex.

You are a professional, but I am an amateur.