What It Means
BTST (Buy Today Sell Tomorrow) is selling shares the very next trading day after you bought them — before they've formally landed in your demat account — a facility Indian brokers offer around the settlement cycle's timing gap.
How It Works
When you buy a share, it isn't credited to your demat account the same day; settlement takes until the next trading day. BTST lets you sell on that next day anyway, relying on the person who sold to you on the buy leg to have delivered on time. If they haven't — a "short delivery" upstream — your own sell trade can fail to deliver, which pushes it into an exchange auction.
Warning
BTST isn't allowed on stocks in the Trade to Trade (T2T) segment, and the rare short-delivery/auction scenario is a real tail risk worth knowing about even though most BTST trades settle without issue.