In lieu of means instead of or in place of something else. When your teacher accepts a drawing in lieu of a written report, she's letting you substitute one assignment for another. When a restaurant offers fruit in lieu of fries, it's giving you a different side dish as a replacement.

The phrase sounds more formal than simply saying “instead of,” which is why you'll often see it in official documents, announcements, or written instructions. A museum might request donations in lieu of an admission fee. A school might give extra recess in lieu of the usual assembly.

You use in lieu of when something serves as a substitute or replacement. If you bring cupcakes in lieu of cookies for the class party, the cupcakes are filling the same role. If someone works on Saturday in lieu of Friday, they're swapping one day for another. The phrase emphasizes that whatever comes after it is taking the place of something originally planned or expected.