person who is deprived of liberty against their will
A prisoner is a person who is forced to stay in a prison against their will. People who are suspected of a crime, for example, a murder, may be arrested and held in jail so they won't escape before their trial. After conviction a prisoner may be punished. For example, they may be imprisoned for a fixed number of years, or receive a life sentence meaning they stay in jail until they die.
A prisoner of war is a person who was captured by the enemy in a war.