(noun)
broadly, a person who is in charge of inspecting things and making sure they are appropriate to the situation, for example, one who oversees a polling station or one who checks the validity of passengers' tickets on buses or trains;
e.g. They showed their tickets to the inspector before taking their seats on the train.
(noun)
an officer on a police force who usually oversees several administrative units but who ranks below the police chief or superintendent;
e.g. She was promoted to police inspector after spending years on the force.