This is the 1897th most frequent French word.
policier
"Policier" means "police officer" in English.
In this context, 'policier' is an adjective modifying 'roman' (novel), describing it as related to crime or detective stories.
Ce roman policier est très captivant.
This detective novel is very captivating.
Here, 'policiers' is a plural adjective describing 'films' (movies), indicating their genre is related to crime or law enforcement.
Il aime les films policiers.
He likes crime movies.
Here, 'policier' is used as a noun, meaning 'police officer', referring to an individual in law enforcement.