Caustic soda is

NaOH is called caustic soda caustic soda is highly deliquescent and hence caustic soda absorbs moisture from atmospheric air.
I) Hygroscopic is the phenomena of absorbing (or) adsorbing of water molecules from atmosphere. Here a dry substance become moist but physical state remain unchanged
Eg: CaO, CuO, sucrose
II) Deliquescent is high degree of hygroscope, the substance absorbs moisture from atmosphere until it dissolve in absorbed water and forms a solution. The vapor pressure of deliquescent substance is less than the partial pressure of water in the atmosphere