A pop quiz was a test given with little warning on a particular subject.
In 2371, Vice admiral Patterson met Kathryn Janeway in a transporter room at the Utopia Planitia Fleet Yards and asked her several science questions, which she answered correctly. He then "ordered" "Katie" to give him a hug, which she did, saying she wasn't expecting a pop quiz. Patterson explained that he wanted her to remember, despite her command position, that she was a scientist first. (VOY: "Relativity")