(n.) an island in the middle of a river
Usually in English, a small island in the middle of a river is known as an ait, or an eyot—both of which are alternative spellings of the same word, and both come from the Old English word for an island, eg. Proving that English can never have enough words for the same thing, though, there’s another (albeit considerably less well-used) word here too: a mediamnis.
While ait and eyot are Old English, mediamnis has unsurprisingly been adopted from Latin. That initial ‘medi–’ comes from the Latin word for ‘middle’, medium, while amnis is a Latin word for a river. Despite that etymology, in Latin this word was more typically used to refer to a canal, a dyke, or some similar manner of watercourse, than it was an area of land in the middle of a body of water. In fact, many Latin dictionaries still record this word as such today.
In English, however, this work went a different direction and its etymological meaning shone through. In the 1500s, the English scholar and antiquarian John Leland hauled it out of relative obscurity and used it (albeit always in its plural form, mediamnes) to refer to various river-central islands in his travel writing; Leland’s journals and descriptions of his travels are now anthologized under the title of his Itinerary, and its in those that the earliest records of this word in English are found.
Perhaps influenced by Leland’s work, later geographers and writers followed suit, and by the eighteenth century the word mediamnes was in occasional use in English as a term for an often fairly sizeable island in the middle of a river.
By the nineteenth century, however, the word had fallen out of favour, and it has remained largely unused in English ever since.