Dry basis is an expression of the calculation in chemistry, chemical engineering and related subjects, in which the presence of water is ignored for the purposes of the calculation. Water is neglected because addition and removal of water are common processing steps, and also happen naturally through evaporation and condensation; it is frequently useful to express compositions on a dry basis to remove these effects.
Example
An aqueous solution containing 2 g of glucose and 2 g of fructose per 100 g of solution contains 2/100=2% glucose on a wet basis, but 2/4=50% glucose on a dry basis. If the solution had contained 2 g of glucose and 3 g of fructose, it would still have contained 2% glucose on a wet basis, but only 2/5=40% glucose on a dry basis.
References
Dry basis Wikipedia(Text) CC BY-SA