Definition. The total amount of BTC that is provably lost, covering coins that fall into one of three categories: unclaimed miner rewards, BTC sent to burn addresses, and BTC sent to OP_RETURN outputs.

Technical. Unclaimed miner rewards arise because the Bitcoin protocol allows the miner of a valid block to claim a reward plus transaction fees, but miners may claim less than the specified amount. A burn address is identified as an address whose private key has provably no owner, such as one virtually impossible to be randomly generated (for example, 1Anything11111111111111111125qfuN). OPRETURN is a script opcode primarily used to store data on the blockchain, and coins sent to OPRETURN are provably unspendable as they do not get added to the UTXO set.