Owing to its large proportion of fibres and its narrow vascular structure, hardwood is a characteristically strong and heavy wood. The term was coined for customs purposes and was used for wood with a darr density of above 0.55 g/cm³ (for example beech, oak, ash). Wood with lower density is called softwood.

The particularly robust and durable, dark wood from the inner core of a trunk (see also sapwood) is called heartwood. Substances such as tannins and pigments, resins and oils are stored in the dead cells of the heartwood. Heartwood trees is a collective term for trees with a core that is a different colour to that of the sapwood, the outer area, for example, oak, cherry and walnut.

Hygroscopicity refers to the ability of certain materials to absorb atmospheric moisture. The characteristic of wood being a hydroscopic material means it can absorb moisture but can also release it again.