1 : to work with another person or group in order to achieve or do something
The two companies agreed to collaborate.
— often + on or in
They collaborated on a book about dogs.
The pair collaborated with each other on the film.
Several doctors collaborated in the project.
— sometimes followed by to + verb
They collaborated to write a book.
2 disapproving : to give help to an enemy who has invaded your country during a war
He was suspected of collaborating with the occupying army.