Ans. 'Leukoplakia.'
Leukoplakia is characterized by development of flat, thickened, white patches, on the lining of mucous membrane of mouth and tongue. The most common cause of leukoplakia is smoking due to chronic irritation of cells from tobacco.
These patches are also known as precancerous lesions as more serious cases can lead to development of oral cancer.
Thus, the correct word to be filled in the blank is 'leukoplakia.'