The word 'isle' entered English in the late thirteenth century from Old French 'isle' (modern French 'île'), descended from Latin 'insula' (island). It is a literary and poetic synonym for 'island,' used in elevated prose, verse, and proper names: the Isle of Man, the Isle of Wight, the British Isles, the Emerald Isle.
The relationship between 'isle' and 'island' is one of the most instructive false etymologies in English. They look related. They mean the same thing. They both contain the sequence i-s-l. But they are etymologically unrelated. 'Isle' is Latin, from 'insula.' 'Island' is Germanic
The 's' was inserted into 'island' in the fifteenth and sixteenth centuries by scribes and scholars who assumed the word must be connected to 'isle' and Latin 'insula.' This was a period when English writers were enthusiastically re-Latinizing their vocabulary, sometimes correctly (restoring the 'b' in 'debt' from Latin 'debitum') and sometimes incorrectly. 'Island' was an incorrect case: the 's' has no historical justification. It has been silent ever since, a permanent monument to a mistaken etymology.
The diminutive 'islet' (a very small island) entered English from French in the sixteenth century. It has been most consequential not in geography but in anatomy: the 'islets of Langerhans' are clusters of hormone-producing cells in the pancreas, named by Paul Langerhans in 1869. From these islets, the hormone 'insulin' took its name in 1922.
French 'île' underwent its own orthographic evolution. The Old French form 'isle' still had the 's,' which was silent by that period. In the eighteenth century, French orthographic reform replaced the silent 's' with a circumflex accent over the vowel — 'île' — a convention used throughout French to mark where an old 's' had disappeared (compare 'hôpital' from 'hospital,' 'forêt' from 'forest'). English, which borrowed the word
The Romance cognates all descend from 'insula' with regular sound changes: Italian 'isola,' Spanish 'isla,' Portuguese 'ilha,' Catalan 'illa,' Romanian 'insulă.' The initial vowel varies but the family is recognizable. The evolution from 'insula' to 'isle' shows typical French phonological processes: loss of the unstressed final vowel, reduction of the consonant cluster, and the gradual silencing of the 's.'
In English literature, 'isle' carries a weight of associations that 'island' does not always bear. Shakespeare's 'Tempest' is set on an unnamed isle. Tennyson wrote of the 'Isle of Avalon.' 'Isle' evokes romance, mystery, enchantment — the distant place across the water where rules are different. 'Island,' by contrast, is neutral and modern, suitable for geography
The word 'isle' thus occupies a peculiar position in English: it is the legitimate Latin-descended term for an island, it accidentally corrupted the spelling of the unrelated Germanic word 'island,' and it survives primarily in literary and ceremonial contexts while its Germanic rival handles the everyday work.