the.com/rational number

means A number that can be written exactly as one integer divided by another (nonzero) integer, like 3/4 or -5/1 — including every whole number and any decimal that ends or repeats.

from From Latin 'ratio' meaning 'reckoning, calculation, relation between quantities' — the same root as 'ratio' itself. A rational number is literally a number expressible as a ratio of integers. The mathematical sense rides on 'ratio,' not on 'reason' in the everyday sense — though both trace to the same Latin ancestor, 'reri,' to think or calculate. Irrational numbers like √2 earned their name for failing this exact test, not for being 'unreasonable,' though the pun has always been irresistible.