chef d'œuvre

Short definition from Wiktionary

Noun

1. 1
   A piece of work that has been given much critical praise, especially one considered the greatest work of a person's career.
2. 2
   A work of outstanding creativity, skill or workmanship.
3. 3
   A work created in order to qualify as a master craftsman and member of a guild.
4. 4
   Alternative form of chef d'œuvre.