A measure of land formerly in use in Wales. Also (later) more generally: an allocated portion of land.
The size of area described varies significantly in different times and places but is frequently likened to an acre..
Origin
Early 18th century. From Welsh erw measure of land, plot of land, field, cognate with Old Cornish erw, ereu field (Middle Cornish erow acre), Middle Breton eru small field (Breton erw, erv, ero raised earth between furrows, measure of land) from an extended form (with w-extension) of the same Indo-European base as Old High German ero earth.