As I understand it, the dry weight of something is its weight minus the weight of its water content. Is this the definition? What about dry cell weight?
According to my Henderson's Dictionary of Biological Terms, dry weight is
The weight or mass of organic matter or soil after removal of water by heating to constant weight.
So yes, your definition is correct and it is also applicable to cells. The dry weight of cells is the weight left when their water content has been removed by heating.