XBT is an alternative code for Bitcoin (BTC), the cryptocurrency covered in full on its own page. Both codes refer to the same asset; this page explains why there are two.
Currencies are usually identified by three-letter codes from the international standard ISO 4217, such as USD for the US dollar or GBP for the pound sterling. The first two letters normally name the country and the third the currency. BTC does not fit that pattern, because "BT" is the country code for Bhutan, so BTC could be mistaken for a Bhutanese currency. Codes for assets that belong to no single country start with X instead: XAU for gold, XAG for silver and XDR for the IMF's Special Drawing Rights. XBT applies the same logic to bitcoin.
XBT has never been formally adopted into ISO 4217, but it has been used by some data providers, exchanges and derivatives venues, including on financial terminals and on some futures contracts. Most retail exchanges and price sites use BTC, which remains far more common. Smaller units have their own names, such as satoshis, but no separate currency code.
The practical rule is simple: wherever XBT appears, it means bitcoin. Traders comparing prices across platforms should simply treat XBT and BTC as identical tickers, while checking separately that a contract is not quoted in a related unit, such as a fraction of a coin or a futures contract with its own size.