Definition - What does BTC1 mean?
BTC1 is a Bitcoin protocol that has split off from two other bitcoin chains: Bitcoin Core and Bitcoin Cash. A series of “forks” have resulted in these three bitcoin projects, which are separate and autonomous.
Techopedia explains BTC1
BTC1 is a bitcoin project that enforces the Segregated Witness protocol or SegWit. SegWit was created to solve a block size problem by appending a signature instead of including it in an original segment. SegWit gained widespread support and is now supported by bitcoin miners.
One way to characterize BTC1 is as a place for the implementation of a hard fork that is not part of Bitcoin Cash or Bitcoin Core (a hard fork related to SegWit protocol). To that end, the Bitcoin Core community is taking care to disavow BTC1 and advise users that BTC1 does “not have anything to do” with Bitcoin Core.