Liz- I took the definition out of a dictionary! Then I looked in Webster's Collegiate. It says: "A plain-woven sheer to coarse cotton fabric." So I look further and find that it is defined differently by the Oxford E. D. "The general name for the most delicately woven cotton fabrics, inc....(listing everything from dresses to curtains (drapes) etc.)".
Another fault line between the 'Old Country' and the 'New World'?
Almost as bad as biscuit and cookie!

Becoming pedantic (after perusing more of the OED), the real (original) muslin was a linen cloth from the Middle East but this definition apparently got lost through the years.

(I use cheesecloth, but for light shade in the greenhouse.)