Merriam-Webster's Word of the Day for August 13, 2025 is:

calculus • \KAL-kyuh-lus\  • noun

Calculus refers to an advanced branch of mathematics that deals mostly with rates of change and with finding lengths, areas, and volumes. The word can also be used more broadly for the act of calculating—that is, estimating something by using practical judgment, or solving or probing the meaning of something.

// By my calculus the more efficient air conditioner will have paid for itself within a span of five years.

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Examples:

“[Manager, Craig] Counsell said that all options are being considered, and the Cubs will wait to make a decision until they have to, as events between now and [Shota] Imanaga’s official return to the active roster could change the calculus.” — Vinnie Duber, The Chicago Sun-Times, 21 June 2025

Did you know?

Solving calculus equations on a chalkboard allows one to erase mistakes, and also hints at the word’s rocky, and possibly chalky, past. Calculus entered English in the 17th century from Latin, in which it referred to a pebble, often one used specifically for adding and subtracting on a counting board. The word thus became associated with computation; the phrase ponere calculos, literally, “to place pebbles,” meant “to carry out a computation.” The Latin calculus, in turn, is thought to perhaps come from the noun calx, meaning “lime” or “limestone,” which is also the ancestor of the English word chalk. Today, in addition to referring to an advanced branch of mathematics, calculus can also be used generally for the act of solving or figuring something out, and as a medical term for the tartar that forms on teeth, among other things. Whichever way it’s used, we think that calculus rocks.