Oral submucous fibrosis is a chronic, complex, premalignant (1% transformation risk) condition of the oral cavity, characterized by juxta-epithelial inflammatory reaction and progressive fibrosis of the submucosal tissues (the lamina propria and deeper connective tissues). As the disease progresses, the oral mucosa becomes fibrotic to the point that the person is unable to open the mouth. The condition is remotely linked to oral cancers and is associated with areca nut and / or its by-products chewing, majorly practiced in South and South-East Asian countries. The incidence of OSMF has also increased in the western countries due to the changing habits and constant migrating population. Definitions
Per Jens J Pindborg and Satyavati Sirsat (1966) (Pathological definition)-
An insidious chronic disease affecting any part of the oral cavity and sometimes the pharynx. Although occasionally preceded by and/or associated with vesicle formation, it is always associated with a juxta-epithelial inflammatory reaction followed by a fibro-elastic change of the lamina propria, with epithelial atrophy leading to stiffness.Per Mohit Sharma and Raghu Radhakrishnan (2019) - An insidious, chronic potentially malignant fibrotic disorder affecting the entire oral cavity and sometimes the pharynx and oesophagus.
Although occasionally preceded by and/or associated with vesicle formation, it is always associated with a juxta-epithelial inflammatory reaction followed by a fibroelastic change of the lamina propria with epithelial atrophy leading to stiffness of the oral mucosa, progressive decrement in mouth opening and inability to eatPer Chandramani More and Naman Rao (2019) (Clinical definition)-
A debilitating, progressive, irreversible collagen metabolic disorder induced by chronic chewing of areca nut and its commercial preparations; affecting the oral mucosa and occasionally the pharynx and esophagus; leading to mucosal stiffness and functional morbidity; and has a potential risk of malignant transformation. Epidemiology
The incidence of the disease is higher in people from certain parts of the world including South and South East Asian, South Africa and the Middle Eastern countries. Symptoms
In the initial phase of the disease, the mucosa feels leathery with palpable fibrotic bands. In the advanced stage the oral mucosa loses its resiliency and becomes blanched and stiff. The disease is believed to begin in the posterior part of the oral cavity and gradually spread outward. Other features of the disease include: 

Xerostomia
Recurrent ulceration
Pain in the ear or deafness
Nasal intonation of voice
Restriction of the movement of the soft palate
A budlike shrunken uvula
Thinning and stiffening of the lips
Pigmentation of the oral mucosa
Dryness of the mouth and burning sensation (stomatopyrosis)
Decreased mouth opening and tongue protrusion

Causes
Dried products such as paan masala and gutkha have higher concentrations of areca nut and appear to cause the disease.