Block time defines the time it takes to mine a block. Both in bitcoin blockchain and ethereum blockchain, there is an expected block time, and an average block time. In bitcoin, the expected block time is 10 minutes, while in ethereum it is between 10 to 19 seconds. Both bitcoin and ethereum, at the time of this writing use a proof of work based distributed consensus algorithm (ethereum is planned to move to a proof of stake based algorithm with its serenity release). The expected block time is set at a constant value to make sure, miners cannot impact the security of the network by adding more computational power. The average block time of the network is evaluated after n number of blocks, and if it is great than the expected block time, then the difficulty level of the proof of work algorithm will be reduced, and if it is less than the expected block time then the difficulty level will be increased. That’s the core design principle behind block time, but you will see as we proceed, how bitcoin and ethereum differentiate themselves from each other.