$BTC

BTC is the abbreviation and commonly used ticker symbol for Bitcoin.  

Here's a more detailed explanation of what Bitcoin is:

Decentralized Digital Currency: Bitcoin is a digital or virtual currency that uses cryptography for security. Unlike traditional fiat currencies issued by governments, Bitcoin operates without a central authority like a bank. Its decentralized nature means transactions are peer-to-peer, occurring directly between users on the Bitcoin network.  
Cryptocurrency: As a cryptocurrency, Bitcoin relies on cryptographic principles to secure and verify transactions, as well as to control the creation of new units.  
Blockchain Technology: Bitcoin's functionality is built upon a technology called the blockchain. This is a distributed, public ledger that records all Bitcoin transactions in a transparent and immutable manner. Each group of verified transactions is added to the blockchain as a "block," which is cryptographically linked to the previous block, forming a chain.  
Limited Supply: One of Bitcoin's fundamental characteristics is its finite supply. The total number of Bitcoins that will ever be mined is capped at 21 million. This scarcity is a key factor in its perceived value as a digital store of value, often compared to "digital gold."