What do you call the plank of wood?

"Plank of wood" is already a perfectly acceptable term. However, you could also use these depending on the context:

General:

* Board: This is a more general term for a flat piece of wood.

* Timber: This refers to wood used for construction.

* Wood: A simple, general term for the material.

Specific:

* Beam: If the plank is long and strong, used for support.

* Slat: If the plank is thin and narrow.

* Battens: If the plank is used to reinforce or strengthen something.

* Panel: If the plank is part of a larger structure or covering.

Descriptive:

* Wooden plank: This clarifies the material.

* Rough-sawn plank: If the plank is unfinished.

* Planed plank: If the plank is smooth and finished.

The best term depends on what you are describing and the intended use of the plank.