in a way that involves or amounts to only a small part or portion of something; by a tiny degree or margin.
From fractional (adjective) + -ly (adverb suffix). Fractional comes from fraction, which traces to Latin fractus (broken) and frangere (to break). The -ly suffix converts the adjective to an adverb indicating manner.
This word appears constantly in scientific writing because scientists care deeply about precise increments—a temperature rising fractionally can mean the difference between a stable climate and a tipping point. It's the language of people who measure the unmeasurable.
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