FOAM: Assembly of cells with solid edges or faces, packed together so that they fill space.
Almost any material can be foamed. Polymers, of course, are the most common ones. However, metals, ceramics, glasses, and even composites can be fabricated into cells.
The most important single feature of a foam is its relative density, ρ*/ ρs; that is, the density of the foam, ρ*, divided by that of the solid from which the cell walls are made, ρs.
Closed or open cells: If the solid of which the foam is made is contained in the cell edges only (so that the cells connect through open faces), the foam is said to be open-celled. If the faces are solid too, so that each cell is sealed off from its neighbours, it is said to be closed-celled. Some foams are partly open and partly closed.