What is a flowable solid?

Answer 1

A flowable solid is said to be plastic. A good example can be seen in old window glass in a house. The tops of the glass often are bent down as the wall above it sags. If you measure the thickness of the glass at the top and then at the bottom, you will find that the glass is measurably thicker at the bottom, the result of the glass "flowing" downward under the effect of gravity.

Answer 2

Flowable solids are materials which are solid, but fragmented into many small chunks and therefore able to move similarly to liquids. Examples would be grain, sand, gravel, table salt, etc. Often mechanical agitation is required to induce the material to flow, but depending on circumstances even slight disturbances can cause sudden movement called avalanches, cascades, etc.

To illustrate, a sheet of lead is completely solid at room temperature. So is a similar sized sheet of glass (which "flows" ten times slower than the lead counterpart). If you then take these two solid sheets and melt the lead into BBs and crush the glass into powder, both materials become flowable solids.