/ˈʌnjən/
[noun] A round vegetable with a strong smell and sharp taste, made up of many layers of thin skin wrapped tightly around each other. It is widely used in cooking all over the world.
The onion gets its name from the Latin word for 'unity' — because its many layers form one perfect whole.
The story of the onion begins in Latin, with the word 'unio', which meant both a large, single pearl and a type of onion. This is a fascinating double meaning: 'unio' came from 'unus', meaning 'one', and the connection was the idea of wholeness — a pearl is a single, smooth, unified object, and the onion, despite its many layers, presents itself to the world as one complete, round thing. This root 'unus' is the same root that gives English words like 'unite', 'unity', and 'unique'.
From Latin 'unio', the word passed into Old French as 'oignon', which is still the modern French word for onion today. The Old French form was the direct bridge into English. When the word arrived in Middle English during the 14th century, it appeared as 'unyoun' or 'oyno(u)n', spellings that show how English speakers were hearing and adapting the French sounds around them. Over the following two centuries, the spelling gradually settled into the familiar 'onion' we use today.
What is remarkable about the word's journey is how stable its meaning has been. Many words shift dramatically in meaning as they travel through languages and centuries, but 'onion' has always referred to this same pungent, layered vegetable. The vegetable itself was well known in the ancient world — Egyptians revered it as a symbol of eternal life, precisely because of those concentric rings that seemed to echo endlessly inward, a circle within a circle within a circle. Roman soldiers ate onions regularly for their supposed strength-giving properties.
The Latin 'unus' that underpins the whole story is one of the most productive roots in the English language. Through Latin 'unire' (to unite) → Old French 'unir' → English 'unite'. Through Latin 'unitas' (oneness) → Old French 'unité' → English 'unity'. Through Latin 'unicus' (one of a kind) → French 'unique' → English 'unique'. Even the word 'union', meaning people joined together as one, follows the same path: Latin 'unio' → Old French 'union' → English 'union'. The onion and the union are, at their etymological heart, the same word.
In informal British English, 'onion' has also taken on a playful slang meaning: it can refer to the head, probably because of the round shape. The phrase 'knowing your onions' — meaning to know your subject well — appeared in early 20th century American English, though its precise origin is debated. Some connect it to the British lexicographer C.T. Onions, who worked on the Oxford English Dictionary, though this link remains uncertain. What is not uncertain is that the humble onion, that everyday kitchen staple, carries within its name one of the great philosophical ideas of the ancient world: that many things, layered together, can make one.