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Block weight is a measure of the size of a block, measured in weight units. The Bitcoin protocol limits blocks to 4 million weight units, restricting the number of transactions a miner can include in a block. Four million weight units is equivalent to 4MB of data, meaning the maximum size for a block is now 4MB. This limit is in place to prevent the blockchain from growing too fast, which would prevent individual users from running a node and fully validating the blockchain, which would in turn harm Bitcoin’s decentralization.
Block weight was implemented as the measure for block limits with the SegWit upgrade. Before Segwit, the block limit was 1MB, as measured in bytes, and was called block size.