I treat definition of this with caution. I was taught the meaning in college and have used it ever since without correction. My tutors explained the differences between strawboard ~ pulpboard ~ greyboard ~ millboard. That listing is in ascending order of quality and price. We had no use for strawboard or pulpboard. At the time the country was still reminded of the wartime years when quality fibres and lengthy processes were rare.
My understanding is that greyboard contains a great deal of recycled fibre ~ newsprint being favourite. This is evident in some batches I have used where little flecks of coloured inks can clearly be seen as impurities ~ expected as part of the specification. The quality of the board is determined by the lengths of the fibres ~ they naturally get broken down as they are recycled. Some paste ca be added to the mix to help them stick together ~ and roller compression in the mill helps firm things up.
Most greyboard manufacturers produce two or three from a range of thicknesses ~ typically 0.5 ~ 1 ~ 1.5 ~ 2 ~ 2.5 ~ and maybe 3mm. They are sold by the thickness rather than the grammage ~ and often by the weight-bundle ~ especially if buying in bulk.
Bulk buying is beneficial where practicable. It enable the end user to choose sizes without waste ~ and the onerous task of cutting to size is a problem the mill is happy to be spared.
I believe that in America such boards are called chipboard. Paperboard and pasteboard are other terms.