Learner's definition of COMPANION
a person or animal you spend time with or enjoy being with
— sometimes used figuratively
something that is meant to be used with something else
— often + to
The book is a companion to the television series with the same name.
The book is the companion volume to his previous book on jazz.
The table is a companion piece to the two chairs.
— used in the titles of books that give information about a particular subject
a person who is paid to live with and help someone who is older or sick