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A vByte is a unit of measure for the weight of blocks and transactions. The vByte was introduced by the SegWit upgrade. A vByte is equivalent to 4 weight units, and thus, a block is limited to being 1 vMegabyte large, or 4 million weight units.
Wallets usually calculate and display transaction fees in terms of sats/vByte, meaning the fee is paid per vByte of data used. Thus, the larger the transaction, the larger the total fee must be to ensure the desired speed of verification. This setup also makes SegWit transactions cheaper than regular transactions, as a byte of Witness data is only equivalent to 1 weight unit (1/4 vByte), while a byte of non-Witness data is 4 weight units (1 vByte).