Prisonnier vs. détenu
The French words prisonnier and détenu both translate to the English word "prisoner", but they carry distinct nuances. While prisonnier is a broad, everyday term that describes anyone deprived of their freedom in physical or metaphorical contexts, détenu is a strictly legal and administrative term referring specifically to an inmate held in a penal institution by the justice system.
Prisonnier
The word prisonnier is the most general term for someone who has been captured or deprived of liberty. It can be used for someone locked in a literal cell, a prisoner of war, a hostage, or even someone metaphorically trapped by their emotions or circumstances.
Le prisonnier s'est échappé de sa cellule au milieu de la nuit.
(The prisoner escaped from his cell in the middle of the night.)
Mon grand-père a été fait prisonnier pendant la Seconde Guerre mondiale.
(My grandfather was taken prisoner during the Second World War.)
Elle se sent prisonnière de ses nombreuses dettes financières.
(She feels like a prisoner to her many financial debts.)
Les ravisseurs exigent une rançon extravagante pour libérer le prisonnier.
(The kidnappers are demanding an extravagant ransom to release the captive.)
Nous sommes tous parfois prisonniers de nos propres habitudes de vie.
(We are all sometimes prisoners of our own life habits.)
Détenu
The word détenu acts as the formal equivalent of "inmate" or "detainee". It is used in legal, journalistic, and administrative contexts to specifically describe a person incarcerated in a prison or holding facility awaiting trial or serving a judicial sentence.
Le nouveau détenu a été immédiatement placé dans le quartier de haute sécurité.
(The new inmate was immediately placed in the high-security ward.)
Les conditions de vie des détenus en prison inquiètent les associations des droits de l'homme.
(The living conditions of the detainees in prison worry human rights organizations.)
Selon le règlement, chaque détenu bénéficie d'une heure de promenade en cour.
(According to the rules, every prisoner receives one hour of recreation in the yard.)
Ce juge a refusé la libération conditionnelle du détenu pour plusieurs raisons.
(This judge denied parole for the inmate for several reasons.)
L'administration pénitentiaire doit gérer un nombre record de détenus cette année.
(The prison administration has to manage a record number of inmates this year.)
Summary
To choose the correct word, remember that prisonnier relates to the broad concept of captivity, whether you are talking about historical contexts, kidnapping, or metaphorical traps. Conversely, détenu belongs to the formal vocabulary of law and journalism, strictly referring to officially registered inmates inside a detention facility.