This question was addressed to me in a job interview.
The query is from Methods and Materials for Degassing in chapter Foundry Introduction & Materials of Casting-Forming-Welding II
For explanation: During production of casting products, gases such as oxygen, hydrogen and nitrogen get dissolved in the casting. The term degassing is employed for the removal of these gases from the casting melt. Dissolved oxygen from molten casting cannot be extract as molecular oxygen, removing of oxygen from melt is called as de-oxidation.