/ˈræʃ.ən.əl/
[adjective] Based on clear thinking and good reasons, rather than on emotions or feelings. A rational person or decision uses logic and facts to reach a conclusion.
'Rational' comes from the Latin word for 'reason' — the same root that gave us 'ratio,' originally meaning a military food allowance.
The story of 'rational' begins with the Latin verb 'reri,' meaning 'to think' or 'to calculate.' From this verb came the noun 'ratio,' which in classical Latin referred to a reckoning, an account, or a calculation — essentially the process of working something out in an orderly way. Roman military writers also used 'ratio' to mean a fixed allowance of food given to soldiers, which is the same 'ratio' we use today when we talk about rationing supplies. The connection between counting things out carefully and dividing them fairly is at the heart of this ancient word.
From 'ratio,' Latin built the adjective 'rationalis,' meaning 'belonging to reason' or 'endowed with reason.' Roman philosophers, particularly Cicero, used 'rationalis' to translate the Greek philosophical term 'logikos' — related to 'logos,' meaning reason or word. This gives 'rational' a direct link to one of the most important ideas in ancient Greek thought: that human beings are distinguished from animals by their capacity for reasoned thought. 'Rationalis' carried a sense of dignity and order — to be rational was to be fully human.
The word passed into Old French as 'rationel' and arrived in English during the late 14th century, when English writers were heavily borrowing from both Latin and French to build a vocabulary for philosophy, theology, and science. Early English uses of 'rational' referred almost always to human beings themselves — a 'rational creature' was simply a person, someone capable of reason, as opposed to beasts or angels. The word described a quality of mind, not just a quality of thought.
Over the following centuries, 'rational' gradually shifted from describing a type of being to describing a type of action or decision. By the 17th and 18th centuries — the period known as the Age of Reason — philosophers such as Descartes, Locke, and Kant were using 'rational' constantly to mean 'governed by reason rather than emotion or superstition.' The word became a badge of Enlightenment values: to be rational was to be modern, scientific, and trustworthy. This was also the era when mathematics adopted 'rational' as a technical term, describing numbers that can be expressed as a simple fraction — a perfectly orderly, calculable quantity.
Perhaps the most surprising aspect of 'rational' is how many everyday English words share its Latin root 'reri' or 'ratio.' Latin 'ratio' → Old French 'raison' → Middle English 'resoun' → modern English 'reason' traces one direct path. Latin 'ratio' also gave us 'ration' (a measured share), 'ratify' (from Latin 'ratum facere,' to make fixed or calculated), and 'rate' (via Medieval Latin 'rata,' meaning a fixed amount). Even the word 'arrange' carries a distant cousin relationship through the idea of setting things in order. Together, these words reveal that the ancient Romans saw thinking, counting, dividing, and organising as fundamentally the same activity — and 'rational' sits at the very centre of that family.