STT, Securities Transaction Tax, is a government tax charged on the value of trades executed on Indian stock exchanges, collected by the broker and shown on your contract note. The rate depends on the segment. At the time of writing, delivery equity attracts 0.1% on both buy and sell, intraday equity 0.025% on the sell side, futures 0.02% on sale and options 0.1% on the sale premium; rates have been raised in recent budgets, so confirm the current schedule. Suppose you buy ₹1,00,000 of SBI for delivery and later sell for ₹1,10,000; STT is ₹100 on purchase and ₹110 on sale. On options that expire in the money and are exercised, STT is charged on the full settlement value, which can be a nasty surprise. STT is a cost of trading and is not refundable, though it reduces taxable business income for traders.