bc1

bc1 is the prefix used for Bech32 Bitcoin addresses on the Bitcoin mainnet. In Bech32, the leading part of the address, known as the human-readable part (HRP), for mainnet is "bc", and the address begins with "bc1" followed by the encoded data. This format was introduced to replace older base58-based addresses for certain types of transactions and to improve error detection and usability.

Bech32 addresses are used with SegWit and its successors. They encode a witness program that specifies how the funds should be spent. On mainnet, common Bech32 addresses begin with bc1q for Pay-to-Witness-Public-Key-Hash (P2WPKH) and Pay-to-Witness-Script-Hash (P2WSH) outputs, while Taproot transactions (witness version 1) use addresses that typically begin with bc1p. The exact characters after "bc1" reflect the witness version and encoding, but all mainnet Bech32 addresses share the bc1 prefix. Testnet Bech32 addresses use a different HRP, such as "tb1".

Bech32 addresses are case-insensitive in specification, but lowercase is strongly recommended to avoid confusion. They require software that supports Bech32 decoding, and most modern wallets, exchanges, and node software provide this support. Some older wallets or services may still rely on legacy address formats or lack Bech32 compatibility.

In summary, bc1 signifies Bitcoin mainnet Bech32 addresses, encompassing current SegWit formats and Taproot addresses, with the exact after-prefix form indicating the specific witness type and version. See also Bech32 (BIP173), Taproot (BIP341/BIP350), and the broader context of Bitcoin address formats.