Botanically, wood is the secondary vascular tissue, xylem; dead at maturity. Made of cellulose impregnated with lignin.
Secondary growth is the process by which plants (e.g. trees) add new vascular tissue (xylem, phloem) each year.
Where strong seasonality, results in growth rings - large cells in spring, smaller in summer.
Can use to age and infer past growth conditions - Dendrochronology.