Merriam-Webster's Word of the Day for May 21, 2026 is:

lacuna • \luh-KOO-nuh\  • noun

Lacuna is a formal word that refers to a gap or blank space in something—in other words, a missing part. When used with respect to biology, lacuna also refers to a small cavity, pit, or discontinuity in an anatomical structure.

// The absence of hemlock pollen from one stretch of the fossil record is a notable lacuna that suggests the tree may have once suffered from some type of blight that nearly wiped out the species.

// An osteocyte is a cell that is isolated in a lacuna of bone.

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Examples:

“At the heart of every biography ... lies a lacuna—something unknowable, no matter how candid or heavily documented the subject, no matter how familiar or diligent the biographer.” — Casey Cep, The New Yorker, 14 Apr. 2025

Did you know?

If you find yourself drawing a blank when it comes to the definition of lacuna, it might help to imagine drawing water instead, ideally from a lake or lagoon. Lacuna, lake, and lagoon all come ultimately from lacus, the Latin word for “lake.” Latin speakers modified lacus into lacuna to form a word meaning “pit,” “gap,” or “pool.” When English speakers borrowed the term in the 17th century, they used it to refer to a figurative gap in or missing portion of something, such as information or text. (Note that lacuna comes with two plural options: the Latin lacunae \luh-KYOO-nee\ or \luh-KOO-nye\, or the anglicized lacunas \luh-KOO-nuz\.) Lagoon, meanwhile, hewed closer to the Latin lacuna, referring first to a shallow sound, channel, or pond near or connected to a larger body of water, and later to a shallow artificial pool or pond.