What is meant by special dividend

How is special dividend different from interim or final dividend??

Generally a Dividend is special or extra ordinary if it is much larger to normal (based on it’s past history of dividend distribution) dividend. Normally companies will distribute these when they have exceptional results or wishes to make changes to it’s financial structure. To note these are atypical events.
In India they were termed as extraordinary only when they are above 10% of the market value of the underlying stock and in any case if dividend is above 10% of market value and if it has options then it’s strike price will be adjusted accordingly. Check this to know how extra ordinary dividends will effect the strike prices.