Beurre Maître d'Hôtel, or Maître d'Hôtel butter, is a compound butter made with butter, parsley, lemon juice, salt and pepper. It is often one of the ingredients in chateaubriand sauce.
See also Maître d'hotel sauce, a sauce with a similar flavor profile.
Word origins
Historically, the maître d'hôtel (or headwaiter) was responsible for important tableside duties in a restaurant, including boning fish and preparing sauces and salad dressings tableside. Beurre Maître d'Hôtel was one of the condiments that would have been prepared, at tableside, by the maître d'hôtel.