John wrote:

I have come across this term in descriptions of wood. Does it refer to
changes with humidity, or something else?


if you Google on "wood stability" you'll come up with pdf
files and other web site information. as wood absorbs
moisture it gets wider and contracts as it loses moisture.
if one side of a board absorbs more than the other side it
is gonna bow. (the dry side "cups") and there are other
issues.

plain sawn boards are less stable than quarter sawn. and so
on and so on.

dave