All 3 Uses of

immortal
in
A Tree Grows in Brooklyn

Johnny had a hankering after immortality which made him a useless dreamer.†
Chpt 10 *

immortality = eternal life (to live forever)

Yet, as she stood there, she knew that years later she would be haunted by the sorrow in the face of that starveling child and that she would wish bitterly that she had said a comforting word then and done something towards the saving of her immortal soul.†
Chpt 18

That afternoon she wrote a story about a little girl who wanted a doll so much that she was willing to give over her immortal soul to Purgatory for eternity if she could have the doll.†
Chpt 27

Definitions:

(1) (immortal)
living or existing forever

or:

someone famous throughout history

or:

someone who will never die -- such as a mythological god
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More rarely, "The Immortals" denotes a military corps of the Persian Empire. The Immortals were so-named because each time a member of the 10,000 man corps was killed or seriously wounded, he was replaced by another man. They are best remembered in western culture for their role in defeating the badly out-numbered Spartans at the Battle of Thermopylae.