Rational
Etymology
From Latin "rationalis," from "ratio" (a reckoning, calculation, reason), from "reri" (to reckon, think). The same "ratio" gives us the mathematical ratio — a reminder that reason was first imagined as a kind of counting.
adjective · /RASH-uh-nul/
Based on reasoning that follows from evidence and consistent logic rather than impulse or emotion.
What it names
Use it to praise thinking that tracks reasons — but notice it describes the process, not the conclusion, which can be rational and still wrong.
Example
The rational move was to cut the losing bet, even though quitting felt like giving up.
Related idea
Distinguish it from reasonable: rational means internally consistent with your premises; reasonable means sensible and proportionate to others. A demand can be perfectly rational and wholly unreasonable. Beware too its shadow, rationalization — reasons invented after the fact to defend a choice already made.
adjective · /RASH-uh-nul/
In mathematics, describing a number that can be expressed as the ratio of two integers.
What it names
a number expressible as a fraction
Example
One-half and −7 are rational numbers; the square root of 2 is not.
Related idea
From ‘ratio,’ not everyday reason — though both descend from Latin “ratio.”