This is the 3898th most frequent Russian word.
взамен
"In place of" or "in exchange for."
Here, 'взамен' indicates a reciprocal action, meaning 'in return.'
Он помог мне с проектом, а взамен попросил о небольшой услуге.
He helped me with the project and, in return, asked for a small favor.
In this sentence, 'взамен' signifies an exchange, meaning 'in exchange for.'
Я подарил ему книгу, а он взамен дал мне ручку.
I gave him a book, and in return, he gave me a pen.
Here, 'взамен' means 'instead of,' referring to substitution.