Book of Life

Definition:

The term "Book of Life" is used to refer to where God has written the names of all the people whom he has redeemed and given eternal life to.

Revelation refers to this book as "the Lamb's Book of Life." This could be translated as "the book of life belonging to Jesus, the Lamb of God." The sacrifice of Jesus on the cross paid the penalty for people's sins so that they can have eternal life through faith in him.
The word for "book" can also mean "scroll" or "letter" or "writing" or "legal document." It may be literal or figurative.