A wain is a type of horse-drawn, load-carrying vehicle, used for agricultural purposes rather than transporting people, for example a haywain. It normally has four wheels but the term has now acquired slightly poetical connotations so is not always used with technical correctness. However, a two-wheeled 'haywain' would be a hay cart, as opposed to a carriage. "Wain" is also an archaic term for chariot.
Builders of wains were known as wainwrights, just as the builders of carts were known as cartwrights. These trades no longer exist, but the terms survive as the surnames of descendants of those practising these crafts.
A wain was the subject of John Constable's 1821 painting The Hay Wain. The painting, which was part of Constable's Gold Medal exhibit to Charles X, depicts a site in Suffolk, near Flatford on the river Stour.