(doc help) What does (linewidth . 40) mean?

[Graham Percival]

from input/regression/markup-word-wrap.ly does
(linewidth . 40)
mean 40% of the pagewidth, or 40 mm, or...?  I'd like to add a note
about linewidth in the doc string for wordwrap and justify, since
setting the linewidth is very important for using these commands.


(example of usage)
\markup {
  \override #'(linewidth . 40)
{  \wordwrap-string #" Om mani padme hum Om mani padme hum Om mani
padme hum Om mani padme hum Om mani padme hum Om mani padme hum Om
mani padme hum Om mani padme hum.