its a white patch or lesion in the oral cavity ,that cannot be scrapped off.it cannot be characterized to any other diseases.
causes:
Tobacco
alcohol
candidiasis
syphillis
deficiency of vitamins like A,B12,C,ect
HSV1 & HPV
other viral infections
chronic irritations
radiations
galvinisms
others causes
+generally it is seen in patients above 30yrs of age.often in males as they tend to be exposed mush more to tobacco products.although nowadays females are equally affected.

Site:oral mucossa,lips,tongue,palate n gingiva n even floor of the mouth.

c\f:
faintly transelucent white patches..
yellow or brown n even black in patients exposed to excessive tobacco products.
lesions can be fissurred,indurated n even palilomatous "unununu" formations..
pumice like surfaces can be seen..or ever hyper keratotic patches can be seen over the surface of the lesion..
"ebbing tide pattern"
considerably thick
...they r generally non-palpable..(cannot be felt)
they can be uniform or non-uniform,they can easily be ulcerated...and speckled..

Changes in celluler level:
incerase in stratum spinosum layer(acanthosis)
Dysplasia
cellular pleomorphism
loss of polarity
and others...
chronic inflammatory cell infiltrations can be seen
candidal hyphae can be seen..

investigations:

histochemistry.
exfoliative cytology
serological tests
others

dd:
lichen planus
candidiasis
keratosis
nevus
burns
erythema types
leukoderma
syphillitic patches


Malignant transformations: hi chance of malignant transformation,if not treated.
treatment:
stoping the habits
reducing trauma by sharp broken edges of tooth.
other irritations
vitamin therapy
cryo
surgical excision.

another form is associated with HIV n EBV (opportunistic) ,called as hairy leukoplakia