The 80ddb deduction of the Income Tax Act is a tax benefit that allows taxpayers to claim a deduction for the expenses incurred on the treatment of certain specified diseases for themselves or their dependents. The deduction amount depends on the age of the patient and the actual amount of expenditure, subject to a maximum limit of Rs. 40,000 for normal citizens and Rs. 1 lakh for senior citizens. The deduction can be claimed by resident individuals or Hindu Undivided Families (HUFs), but not by non-resident Indians (NRIs). The taxpayer has to obtain a certificate from a specialist doctor as per the prescribed format and rules. The list of specified diseases covered under this deduction includes neurological diseases, malignant cancers, AIDS, chronic renal failure, and hematological disorders.