A billet-doux is a love letter. The word's plural is billets-doux.

// Since their relationship was long-distance, the couple thought it would be cute to send each other billets-doux on their anniversary.

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"... [Jon] Batiste creates splendid music—those of us with kids likely experienced it lately on his soundtrack to the Pixar hit 'Soul.' 'Batiste: Movement 11',' the composition of his that’s nominated for a Grammy in a classical category, is nearly two minutes of intense, pensive joy tinctured with darkness, of the kind that people send to each other as a billet-doux …" — John McWhorter, The New York Times, 25 Feb. 2022

When love is in the air, it's time to put it down on paper. If you ever find yourself having trouble thinking of Valentine's Day or anniversary gift ideas, how about sending your sweetie pie a billet-doux that reminds them of your perfect meet-cute, or invites them to see the latest rom-com? Wouldn’t that be sweet? In French, billet doux means "sweet letter." English writers first fell in love with the word during the 17th century and have been committed to using it as a romantic alternative to "love letter" ever since.