What is the word of meaning timber that partly prepared into planks?

While "lumber" can refer to raw timber, it more commonly refers to timber that has been cut into planks, beams, or other shapes.

Here's why it's not quite "partly prepared":

* Planks are a specific type of lumber that has been processed to a particular dimension (thickness, width).

* Timber is the raw, uncut wood from a tree.

So, "lumber" encompasses the stage of processing that goes beyond just cutting into planks. It includes further preparation like smoothing, drying, and sometimes even treating with preservatives.