The process of oxidation of oil and fat into fatty acids present in food is called Rancidity. ' A substance has gone rancid ' means the fat and oil  present in the substance is oxidised .
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''A substance has gone rancid''  ;-

It is one of the method used in food in which  fats and oil are transformed into fatty acids. This method is known as Rancidity. Any food gets damaged is  known as rancid.

Spoiling of food in the presence of oxygen is known as Rancidity.