The expatriate U.S. artist James McNeill Whistler (1834–1903) was an influential painter and printmaker and an important link between London, Paris, and New York. The infinite shades of half-light depicted in his work inspired a wide range of nuanced terminology. This essay discusses his twilight subjects in the words of the artist himself and his contemporaries, from the crepuscules of Trouville and Valparaíso in the 1860s to the Venetian nocturnes of 1879–80, with a focus on works in The Frick Collection in tribute to the curator emerita Susan Grace Galassi. When Whistler thanked his patron Frederick R. Leyland (1832–1892) for the term nocturne to describe his moonlight scenes, he said that the word “does so poetically say all I want to say and no more than I wish!” The new titles, applied to his twilight subjects as well as his night scenes, mirrored the repositioning of his aesthetic identity. The virtual transformation of his early realistic or impressionistic oils by the imposition of abstract titles confirmed his position as a proponent of “Art for Art’s Sake” and a leading figure in the Aesthetic movement.