警探(jǐng tàn) - detective, police detective in Mandarin

警探 is a detective within the police, the rank that investigates rather than patrols. Its high frequency owes a good deal to subtitles: crime drama is where most learners will meet it, and it renders the English "Detective" used as a title before a name.

The neighbouring words split along a clear line. 警察 is a police officer generally, 侦探 is a detective in the investigating sense and covers the private sort, so Sherlock Holmes is a 侦探 and not a 警探. Put the two together and 私家侦探 is a private investigator, which no 警探 can be by definition.

The measure word for people in an official capacity is 名 or 位, so 一名警探 in reporting and 那位警探 when you want to be polite. As a title it goes before the surname, as in 罗警探.