literally: chief piece of work (French) pronounced "shay duvrə"
A masterpiece, especially one produced by a musician, writer, or artist. Refers to a creation that has been given much critical praise, used for an exceptionally good piece of creative work or the best piece of work of a particular artist or craftsman.
He regarded that particular speech as his chef-d'oeuvre.
This song is a chef-d'œuvre.
Leonardo Da Vinci's Mona Lisa is a chef-d'œuvre.