That is some square timber, or perhaps rectangular, about 20x30.
When talking about types of timber, the dimensions are referred to, traditionally in inches, hence stud-work might be build with "two-by-four", but more commonly nowadays in millimeters.
Other words, while talking about wood: A dowel has a circular cross section. Slats are thin planks, used to cover something. Veneer is paper-thin that is stuck onto the surface of, for example, a tabletop. Lumber and timber have similar meanings. In British English timber tends to be more processed; lumber could be logs or roughly sawn wood; timber might be dried, cut, and planed smooth. In the US, exactly the opposite!