Muslin is a "plain-woven cotton fabric made in various weights. The better qualities of muslin are fine and smooth in texture and are woven from evenly spun warps and wefts, or fillings. They are given a soft finish, bleached or piece-dyed, and are sometimes patterned in the loom or printed. The coarser varieties are often of irregular yarns and textures, bleached, unbleached, or piece-dyed and are generally finished by the application of sizing. Grades of muslin are known by such names as book, mull, swiss, and sheeting." Encyclopedia Britannica
When people say muslin I think of 'cheap' fabric where the quality is poor to match the price and it usually is unbleached cotton. It's a fabric I use when I am making a dress out of really expensive fabric so I first make it in muslin to test the pattern to make sure of any alterations I need to make before cutting into expensive fabric.
BUT is seems according to the Encyclopedia Britannica it could be any plain woven cotton fabric I guess that means most if not all of the 'cotton' fabric we use for quilting is considered muslin. Just that some are in better grades than others.