Too many to count; an extremely large or indefinite number of things.
From Latin 'innumerabilis' (in- not + numerabilis countable). The word has been in English since the 1300s and comes directly from Latin, used for things so plentiful that counting them is impossible or impractical.
Scientists actually use 'innumerable' when referring to atoms in a single grain of sand—but the number isn't truly infinite, just so large (about 10,000 grains of sand have more atoms than stars in the observable universe) that it dwarfs human comprehension. Language breaks down at cosmic scales.
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