BLEU (bilingual evaluation understudy) is an algorithm for evaluating the quality of text which has been machine-translated from one natural language to another. Quality is considered to be the correspondence between a machine's output and that of a human: "the closer a machine translation is to a professional human translation, the better it is" – this is the central idea behind BLEU. BLEU was one of the first metrics to achieve a high correlation with human judgements of quality, and remains one of the most popular automated and inexpensive metrics.

Bleu or BLEU may refer to:

• the French word for blue
• Three Colors: Blue, a 1993 movie
• BLEU (Bilingual Evaluation Understudy), a machine translation evaluation metric
• Belgium–Luxembourg Economic Union
• Bleu (musician), a member of pop-group L.E.O.
• Blue cheese, a type of cheese
• Deis, a character from the Breath of Fire role-playing videogame series who is known as "Bleu" in the English versions
• Corbin Bleu, an American actor, model, dancer and vocalist
• Parti bleu, 19th century political group in Quebec, Canada
• Bleu, synonymous with "extra rare", indicating a barely-cooked meat preparation; very red and cold