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

assiduous • \uh-SIJ-uh-wus\  • adjective

Assiduous is a formal word that means “showing great care, attention, and effort.”

// Thanks to the assiduous efforts of the local land trust over many years, a substantial amount of whip-poor-will habitat is now protected from development.

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Examples:

“My mom was also assiduous about what we would today call food safety, avoiding anything that could possibly cause illness, especially raw meat: ‘Well done’ was the norm for everything, and anything that could possibly go bad was kept in the fridge.” — James Martin, Work in Progress: Confessions of a Busboy, Dishwasher, Caddy, Usher, Factory Worker, Bank Teller, Corporate Tool, and Priest, 2026

Did you know?

While assiduous means “showing great care, attention, and effort,” and in some situations may be an appropriate substitute for careful, it’s got a bit more oomph than careful in that it suggests a dogged or tireless persistence. If you are assiduous in your efforts (or work, research, analysis, training, preparations, etc.) for example, it’s implied that you’re in it for the long haul, or that you have the ability to “sit with” a task or challenge for a considerable amount of time. This idea is fitting given that assiduous comes from the Latin verb assidēre, meaning “to sit beside.”