In a manner that can be counted or enumerated; used to modify verbs or adjectives to indicate that something has the property of being countable.
From 'countable' plus the adverb-forming suffix '-ly'. The adverbial form allows the countability concept to modify actions or qualities in sentences.
You'll mostly see this in mathematics and academic writing—'countably infinite' is a term that describes sets of numbers that seem infinite but can theoretically be listed one by one, unlike the uncountably infinite real numbers!
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