Definition of "polster"

It is on this rocky surface that one finds many of the growth forms associated with extreme life conditions: polsters, mat plants, espaliers.

1915 April, F. J. Smiley, “The alpine and subapline vegetation of the Lake Tahoe region”, in Botanical Gazette, volume 59, page 265

The polsters give the glacier the appearance of a cobble-strewn surface.

1972, Calvin J. Heusser, “Polsters of the moss Drepanocladus berggrenii on Gilkey Glacier, Alaska”, in Torreya‎, volume 99, number 1, pages 34–36

Moss polsters were sampled using an equivalent surface cross-section area to that of the traps.

1991, P. R. Cundill, “Comparisons of moss polster and pollen trap data: a pilot study”, in Grana, volume 30, number 2