A maître d' was the head waiter of a restaurant.
One such maître d' was Sebastian Lassiter, who worked for the part-restaurant and part-time machine the Crystal Bucephalus. When the time travel capacity of the business was injured, he refused to shut down the restaurant as it could negatively impact the business. He was later killed by Kamelion when the robot was being controlled by Maximillian Arrestis. (PROSE: The Crystal Bucephalus)
Another maître d', known as Flemming, worked aboard the starship Harmony and Redemption. As River Song regularly visited the restaurant, he became well acquainted with her and her work. Being an archaeologist, River Song was fully aware that this restaurant would at some point be hit by a meteor, and so when Flemming betrayed her, she knew exactly where to stand in order to survive the meteor strike, which then killed the crew and diners. (TV: The Husbands of River Song)