These days, the term Greyboard is used to describe a low grade, 100% recycled, grey coloured thick board used for pad backing, rigid boxes, carton (not corrugated) toy packaging and bookbinding. The product 'greyboard' which is now manufactured has many forebears and the names of these products are sometimes still referred to. In particular, names which people often refer to are strawboard (it used to be made using cereal straw), unlined chipboard, millboard, container board, Dutch greyboard etc etc.
...and this is what it looks like:
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Although all paper and board is sold by price which is calculated by weight (grams per square meter - gsm) Greyboard and other boards are manufactured to a thickness or caliper, which is measured in microns, often referred to as mics or by the symbol µ (classical Greek for the letter M)
|click on image to enlarge|
|Click on image to enlarge|
Another issue, is that because it is made with the lowest grade of waste, it is not guaranteed to be acid free, and can (not will) discolour and fade, as you can see in the picture below.
For interest, here is a link to a greyboard manufacturer in the US, but they still call it Chipboard over there! http://www.newmanpaperboard.com/#