Fibre-saturation point is the maximum amount of water that can be taken up from the vapour phase at a given temperature by a unit weight of wood or cellulose. It is related to volumetric swelling and o. d. wood density as-
L = a(v)/a(o), where L is FSP, and a(v) and a(o) are volumetric swelling and o.d. wood density respectively.
If there is just enough water to saturate wood cell-wall and no liquid water is present in the cell-lumen, the wood is said to be at its Fibre-Saturation Point(FSP). Water is held on cell-wall and in between micro-fibrils at H-bonding site.

Fibre saturation point is a term used in wood mechanics and especially wood drying, to denote the point in the drying process at which only water bound in the cell walls remains - all other water, called free water, having been removed from the cell cavities. Further drying of the wood results in strengthening of the wood fibres, and is usually accompanied by shrinkage. Wood is normally dried to a point where it is in equilibrium with the atmospheric moisture content or relative humidity, and since this varies so does the equilibrium moisture content.