noun

• (dated) The mistress of a castle or large household.
  "Gerard returned safely to his native land, and in the fantastic spirit of the times, he approached his own castle disguised as a pilgrim, intending to himself and his Chatelaine, the delight of a romantic surprise. […] He accordingly repaired, in the silent hour of midnight, when the castle was wrapped in sleep, to the well-remembered chamber of the Chatelaine. She was not alone. A handsome young page supplied the place of her long-absent lord. In the excess of his rage, Gerard killed his wife on the spot, but reserved the page for a more lingering revenge."
• (historical) A chain or clasp worn at the waist by women with handkerchief, keys, etc., attached, supposed to resemble the chain of keys once worn by medieval chatelaines.
  "This elegant little appendage to the dress, large enough to contain a handkerchief and purse, is deserving of being as generally adopted in the United States as it has recently been in Paris. […] Both sides of the Chatelaine are embroidered alike. […] The hook attaches this Chatelaine to the wristband."